Semiconductor device

ABSTRACT

To provide a semiconductor device which can be miniaturized or highly integrated. To obtain a semiconductor device including an oxide semiconductor, which has favorable electrical characteristics. To provide a highly reliable semiconductor device including an oxide semiconductor, by suppression of a change in its electrical characteristics. The semiconductor device includes an island-like oxide semiconductor layer over an insulating surface; an insulating layer surrounding a side surface of the oxide semiconductor layer; a source electrode layer and a drain electrode layer in contact with top surfaces of the oxide semiconductor layer and the insulating layer; a gate electrode layer overlapping with the oxide semiconductor layer; and a gate insulating layer between the oxide semiconductor layer and the gate electrode layer. The source electrode layer and the drain electrode layer are provided above the top surface of the oxide semiconductor layer. The top surface of the insulating layer is planarized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, a manufacturingmethod, a process, a machine, a manufacture, or a composition of matter.In particular, the present invention relates to a semiconductor device,a display device, a light-emitting device, a method for driving them, ora method for fabricating them, for example.

In this specification, a “semiconductor device” generally refers to adevice which can function by utilizing semiconductor characteristics; anelectrooptic device, a light-emitting display device, a semiconductorcircuit, and an electronic device are all included in semiconductordevices.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. Such a transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (also simply referred to as a display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a technique for forming a transistor using zinc oxide or anIn—Ga—Zn-based oxide semiconductor as an oxide semiconductor isdisclosed (see Patent Documents 1 and 2).

In recent years, demand for an integrated circuit in which transistorsand the like are integrated with high density has risen with reductionsin the size and weight of an electronic device.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide asemiconductor device which can be miniaturized or highly integrated.

Another object of one embodiment of the present invention is to obtain asemiconductor device including an oxide semiconductor, which hasfavorable electrical characteristics.

A further object of one embodiment of the present invention is toinhibit a change in electrical characteristics of a semiconductor deviceincluding an oxide semiconductor. A still further object of oneembodiment of the present invention is to provide a highly reliablesemiconductor device. A yet still further object of one embodiment ofthe present invention is to provide a novel semiconductor device. A yetstill further object of one embodiment of the present invention is toprovide a semiconductor device which does not easily become normally on.A yet still further object of one embodiment of the present invention isto provide a semiconductor device in which a leakage current is noteasily increased. A yet still further object of one embodiment of thepresent invention is to provide a semiconductor device in which athreshold voltage is not easily changed. A yet still further object ofone embodiment of the present invention is to provide a semiconductordevice in which a threshold voltage is not easily degraded. A yet stillfurther object of one embodiment of the present invention is to providea semiconductor device which is not easily affected by a short channeleffect. A yet still further object of one embodiment of the presentinvention is to provide a semiconductor device in which a source regionand a drain region are not easily short-circuited. A yet still furtherobject of one embodiment of the present invention is to provide asemiconductor device which is not easily affected by a variation inchannel length.

Note that the descriptions of these objects do not disturb the existenceof other objects. Note that there is no need to achieve all of theseobjects with one embodiment of the present invention. Other objects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a semiconductor deviceincluding an island-like oxide semiconductor layer over an insulatingsurface; an insulating layer surrounding a side surface of the oxidesemiconductor layer; a source electrode layer and a drain electrodelayer in contact with a top surface of the oxide semiconductor layer anda top surface of the insulating layer; a gate electrode layeroverlapping with the oxide semiconductor layer; and a gate insulatinglayer between the oxide semiconductor layer and the gate electrodelayer. The source electrode layer and the drain electrode layer areprovided above the top surface of the oxide semiconductor layer. The topsurface of the insulating layer is planarized.

It is preferable that the semiconductor device of one embodiment of thepresent invention further include an oxide layer between the oxidesemiconductor layer and the insulating layer, and the oxide layer be incontact with the side surface of the oxide semiconductor layer.

In the semiconductor device of one embodiment of the present invention,it is preferable that the source electrode layer include a first sourceelectrode layer and a second source electrode layer stacked over thefirst source electrode layer, the drain electrode layer include a firstdrain electrode layer and a second drain electrode layer stacked overthe first drain electrode layer, the first source electrode layer andthe first drain electrode layer be in contact with the top surface ofthe oxide semiconductor layer and the top surface of the insulatinglayer, and the second source electrode layer and the second drainelectrode layer be in contact with the top surface of the oxidesemiconductor layer.

It is preferable that the semiconductor device of one embodiment of thepresent invention further include a first oxide layer in contact with abottom surface of the oxide semiconductor layer, and the insulatinglayer surround a side surface of the first oxide layer.

It is preferable that the semiconductor device of one embodiment of thepresent invention further include a second oxide layer in contact withthe top surface of the oxide semiconductor layer, a top surface of thesource electrode layer, and a top surface of the drain electrode layer.

In one embodiment of the present invention, a semiconductor device whichcan be miniaturized or highly integrated can be provided.

In one embodiment of the present invention, a semiconductor deviceincluding an oxide semiconductor can have favorable electricalcharacteristics.

In one embodiment of the present invention, a highly reliablesemiconductor device including an oxide semiconductor can be provided bysuppression of a change in its electrical characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate a structural example of a transistor of oneembodiment.

FIGS. 2A and 2B illustrate structural examples of a transistor of oneembodiment.

FIGS. 3A to 3C illustrate structural examples of a transistor of oneembodiment.

FIG. 4 illustrates a structural example of a transistor of oneembodiment.

FIGS. 5A and 5B illustrate structural examples of a transistor of oneembodiment.

FIGS. 6A to 6E illustrate an example of a method for fabricating atransistor of one embodiment.

FIGS. 7A to 7C illustrate an example of a method for fabricating atransistor of one embodiment.

FIG. 8 illustrates an example of a method for fabricating a transistorof one embodiment.

FIGS. 9A to 9D illustrate an example of a method for fabricating atransistor of one embodiment.

FIGS. 10A to 10E illustrate an example of a method for fabricating atransistor of one embodiment.

FIG. 11 is a schematic diagram illustrating a stacked-layer structure ofa transistor of one embodiment.

FIGS. 12A and 12B illustrate a band structure of an oxide stack of oneembodiment.

FIGS. 13A and 13B illustrate a band structure of an oxide stack of oneembodiment.

FIG. 14 illustrates a band structure of an oxide stack of oneembodiment.

FIGS. 15A and 15B are a circuit diagram and a cross-sectional viewillustrating a semiconductor device of one embodiment.

FIGS. 16A and 16B are a circuit diagram and a cross-sectional viewillustrating a semiconductor device of one embodiment.

FIG. 17 is a cross-sectional view of a semiconductor device of oneembodiment.

FIG. 18 is a block diagram illustrating an example of a semiconductordevice of one embodiment.

FIGS. 19A to 19C are block diagrams illustrating an example of asemiconductor device of one embodiment.

FIGS. 20A to 20C illustrate examples of a semiconductor device of oneembodiment.

FIG. 21 shows relationship between a composition and a depositioncondition of a titanium nitride film of one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to theaccompanying drawings. Note that the invention is not limited to thefollowing description, and it will be easily understood by those skilledin the art that various changes and modifications can be made withoutdeparting from the spirit and scope of the invention. Therefore, theinvention should not be construed as being limited to the description inthe following embodiments. Note that in the structures of the inventiondescribed below, the same portions or portions having similar functionsare denoted by the same reference numerals in different drawings, anddescription of such portions is not repeated. Further, the same hatchingpattern is applied to portions having similar functions, and theportions are not especially denoted by reference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that in this specification and the like, ordinal numbers such as“first” and “second” are used in order to avoid confusion amongcomponents and do not limit the number.

Note that functions of the source and the drain may be switched in thecase where transistors of different polarities are employed or in thecase where the direction of a current flow changes in a circuitoperation. Therefore, the terms “source” and “drain” can be used todenote the drain and the source, respectively, in this specification.

(Embodiment 1)

In this embodiment, a stacked-layer structure of a semiconductor deviceof one embodiment of the present invention will be described withreference to FIG. 11.

The stacked-layer structure of the semiconductor device includes anoxide stack 404 between an insulating layer 402 and a gate insulatinglayer 410. Further, the oxide stack 404 includes a first oxide layer 404a, an oxide semiconductor layer 404 b, and a second oxide layer 404 c.

Each of the first oxide layer 404 a and the second oxide layer 404 c isan oxide layer containing one or more kinds of metal elements which formthe oxide semiconductor layer 404 b.

The oxide semiconductor layer 404 b includes a layer represented by anIn-M—Zn oxide, which contains at least indium, zinc, and M (M is a metalelement such as Al, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf). The oxidesemiconductor layer 404 b preferably contains indium because the carriermobility of a transistor is increased.

The first oxide layer 404 a under the oxide semiconductor layer 404 bincludes an oxide layer which is represented by an In-M—Zn oxide (M is ametal element such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf) and inwhich the atomic ratio of M to In is higher than that in the oxidesemiconductor layer 404 b. Specifically, the amount of any of the aboveelements in the first oxide layer 404 a in an atomic ratio is 1.5 timesor more, preferably 2 times or more, further preferably 3 times or moreas much as that in the oxide semiconductor layer 404 b in an atomicratio. Any of the above elements is more strongly bonded to oxygen thanindium, and thus has a function of suppressing generation of an oxygenvacancy in the oxide layer. That is, an oxygen vacancy is more unlikelyto be generated in the first oxide layer 404 a than in the oxidesemiconductor layer 404 b.

Further, in a manner similar to the first oxide layer 404 a, the secondoxide layer 404 c over the oxide semiconductor layer 404 b includes anoxide layer which is represented by an In-M—Zn oxide (M is a metalelement such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf) and in whichthe atomic ratio of M to In is higher than that in the oxidesemiconductor layer 404 b. Specifically, the amount of any of the aboveelements in the second oxide layer 404 c in an atomic ratio is 1.5 timesor more, preferably 2 times or more, further preferably 3 times or moreas much as that in the oxide semiconductor layer 404 b in an atomicratio.

In other words, when each of the first oxide layer 404 a, the oxidesemiconductor layer 404 b, and the second oxide layer 404 c is anIn-M—Zn oxide containing at least indium, zinc, and M (M is a metalelement such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and the firstoxide layer 404 a has an atomic ratio of In to M and Zn which isx₁:y₁:z₁, the oxide semiconductor layer 404 b has an atomic ratio of Into M and Zn which is x₂:y₂:z₂, and the second oxide layer 404 c has anatomic ratio of In to M and Zn which is x₃:y₃:z₃, each of y₁/x₁ andy₃/x₃ is preferably larger than y₂/x₂. Each of y₁/x₁ and y₃/x₃ is 1.5times or more as large as y₂/x₂, preferably 2 times or more, furtherpreferably 3 times or more as large as y₂/x₂. At this time, when y₂ isgreater than or equal to x₂ in the oxide semiconductor layer 404 b, atransistor can have stable electrical characteristics. However, when y₂is 3 times or more as large as x₂, the field-effect mobility of thetransistor is reduced; accordingly, y₂ is preferably smaller than 3times x₂.

Note that when the first oxide layer 404 a is an In-M—Zn oxide, it ispreferable that, in the atomic ratio between In and M, the proportion ofIn be less than 50 atomic % and the proportion of M be greater than orequal to 50 atomic %, and it is further preferable that, in the atomicratio between In and M, the proportion of In be less than 25 atomic %and the proportion of M be greater than or equal to 75 atomic %. Whenthe oxide semiconductor layer 404 b is an In-M—Zn oxide, it ispreferable that, in the atomic ratio between In and M, the proportion ofIn be greater than or equal to 25 atomic % and the proportion of M beless than 75 atomic %, and it is further preferable that, in the atomicratio between In and M, the proportion of In be greater than or equal to34 atomic % and the proportion of M be less than 66 atomic %. When thesecond oxide layer 404 c is an In-M—Zn oxide, it is preferable that, inthe atomic ratio between In and M, the proportion of In be less than 50atomic % and the proportion of M be greater than or equal to 50 atomic%, and it is further preferable that, in the atomic ratio between In andM, the proportion of In be less than 25 atomic % and the proportion of Mbe greater than or equal to 75 atomic %.

The constituent elements of the first oxide layer 404 a and the secondoxide layer 404 c may be different from each other, or their constituentelements may be the same at the same atomic ratios or different atomicratios.

For the first oxide layer 404 a, the oxide semiconductor layer 404 b,and the second oxide layer 404 c, for example, an oxide semiconductorcontaining indium, zinc, and gallium can be used.

The thickness of the first oxide layer 404 a is greater than or equal to3 nm and less than or equal to 100 nm, preferably greater than or equalto 3 nm and less than or equal to 50 nm. The thickness of the oxidesemiconductor layer 404 b is greater than or equal to 3 nm and less thanor equal to 200 nm, preferably greater than or equal to 3 nm and lessthan or equal to 100 nm, further preferably greater than or equal to 3nm and less than or equal to 50 nm.

It is preferable that each of the first oxide layer 404 a and the secondoxide layer 404 c contain one or more kinds of metal elements formingthe oxide semiconductor layer 404 b and be formed using an oxidesemiconductor whose energy at the bottom of the conduction band ishigher than that of the oxide semiconductor layer 404 b by 0.05 eV ormore, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and is closeto the vacuum level by 2 eV or less, 1 eV or less, 0.5 eV or less, or0.4 eV or less.

In such a structure, when a voltage is applied to a gate electrode layeroverlapping with the oxide stack 404 with the gate insulating layer 410provided therebetween, a channel is formed in the oxide semiconductorlayer 404 b of the oxide stack 404, whose energy at the bottom of theconduction band is the lowest. In other words, the second oxide layer404 c is formed between the oxide semiconductor layer 404 b and the gateinsulating layer 410, whereby a structure in which the channel of thetransistor is not in contact with the gate insulating layer 410 can beobtained.

[Band Structure of Oxide Stack]

A band structure of the oxide stack 404 is described. A stackcorresponding to the oxide stack 404 in which an In—Ga—Zn oxide havingan energy gap of 3.15 eV is used as a layer corresponding to each of thefirst oxide layer 404 a and the second oxide layer 404 c and an In—Ga—Znoxide having an energy gap of 2.8 eV is used as a layer corresponding tothe oxide semiconductor layer 404 b is fabricated, and the bandstructure thereof is analyzed. Note that for convenience, the stack isreferred to as an oxide stack, and the layers forming the stack arereferred to as a first oxide layer, an oxide semiconductor layer, and asecond oxide layer.

The thickness of each of the first oxide layer, the oxide semiconductorlayer, and the second oxide layer was 10 nm. The energy gap was measuredwith the use of a spectroscopic ellipsometer (UT-300 manufactured byHORIBA Jobin Yvon). Further, the energy gap in the vicinity of aninterface between the first oxide layer and the oxide semiconductorlayer was 3 eV, and the energy gap in the vicinity of an interfacebetween the second oxide layer and the oxide semiconductor layer was 3eV.

In FIG. 12A, the energy difference between a vacuum level and a top of avalence band of each layer was measured while the oxide stack was etchedfrom the second oxide layer side, and was plotted. The energy differencebetween the vacuum level and the top of the valence band was measuredusing an ultraviolet photoelectron spectroscopy (UPS) device (VersaProbemanufactured by ULVAC-PHI, Inc.).

In FIG. 12B, an energy difference (electron affinity) between the vacuumlevel and a bottom of a conduction band of each layer, which wascalculated by subtracting the energy gap of each layer from the energydifference between the vacuum level and the top of the valence band, wasplotted.

Part of the band structure in FIG. 12B is schematically shown in FIG.13A. FIG. 13A shows the case where silicon oxide films are provided incontact with the first oxide layer and the second oxide layer. Here, Evrepresents energy of the vacuum level, EcI1 and EcI2 represent energy atthe bottom of the conduction band of the silicon oxide film, EcS1represents energy at the bottom of the conduction band of the firstoxide layer, EcS2 represents energy at the bottom of the conduction bandof the oxide semiconductor layer, and EcS3 represents energy at thebottom of the conduction band of the second oxide layer.

As shown in FIG. 13A, the energies at the bottoms of the conductionbands of the first oxide layer, the oxide semiconductor layer, and thesecond oxide layer are changed continuously. This can be understood alsofrom the fact that the compositions of the first oxide layer, the oxidesemiconductor layer, and the second oxide layer are close to each otherand oxygen is easily diffused.

Note that although the case where the first oxide layer and the secondoxide layer are oxide layers having the same energy gap is shown in FIG.13A, the first oxide layer and the second oxide layer may be oxidelayers having different energy gaps. For example, part of the bandstructure in the case where EcS1 is higher than EcS3 is shown in FIG.13B. Alternatively, although not shown in FIGS. 13A and 13B, EcS3 may behigher than EcS1.

According to FIGS. 12A and 12B and FIGS. 13A and 13B, the oxidesemiconductor layer of the oxide stack serves as a well and a channel ofthe transistor including the oxide stack is formed in the oxidesemiconductor layer. Note that since the energies at the bottoms of theconduction bands are changed continuously, the structure of the oxidestack can also be referred to as a U-shaped well. Further, a channelformed to have such a structure can also be referred to as a buriedchannel.

Since each of the first oxide layer 404 a and the second oxide layer 404c is an oxide layer containing one or more kinds of metal elementsforming the oxide semiconductor layer 404 b, the oxide stack 404 canalso be referred to as an oxide stack in which layers containing thesame main components are stacked. The oxide stack in which layerscontaining the same main components are stacked is formed not simply bystacking layers but so as to have a continuous energy band (here, inparticular, a well structure having a U shape in which energies at thebottoms of the conduction bands are changed continuously between any ofthe layers). This is because when impurities which form a defect statesuch as a trap center or a recombination center are mixed at eachinterface, the continuity of the energy band is lost, and thus carriersare trapped or disappear by recombination at the interface.

In order to form a continuous energy band, the layers need to be stackedsuccessively without exposure to the air with the use of a multi-chamberdeposition apparatus (e.g., a sputtering apparatus) including a loadlock chamber. Each chamber in the sputtering apparatus is preferablysubjected to high vacuum evacuation (to a vacuum of about 1×10⁻⁴ Pa to5×10⁻⁷ Pa) with the use of a suction vacuum evacuation pump such as acryopump in order to remove water or the like which is an impurity foran oxide semiconductor as much as possible. Alternatively, aturbo-molecular pump is preferably used in combination with a cold trapso that a gas does not flow backward from an evacuation system to achamber.

In order to obtain a highly purified intrinsic oxide semiconductor, achamber needs to be subjected to high vacuum evacuation, and inaddition, a sputtering gas needs to be highly purified. When a highlypurified gas having a dew point of −40° C. or lower, preferably −80° C.or lower, further preferably −100° C. or lower is used as an oxygen gasor an argon gas used as a sputtering gas, moisture or the like can beprevented from entering an oxide semiconductor as much as possible.

The first oxide layer 404 a and the second oxide layer 404 c which areprovided over and under the oxide semiconductor layer 404 b each serveas a barrier layer and can prevent a trap level formed at an interfacebetween the oxide stack 404 and each of the insulating layers which arein contact with the oxide stack 404 (the insulating layer 402 and thegate insulating layer 410) from adversely affecting the oxidesemiconductor layer 404 b which serves as a main carrier path for thetransistor.

For example, oxygen vacancies contained in the oxide semiconductor layerappear as localized states in deep energy area in the energy gap of theoxide semiconductor. A carrier is trapped in such localized states, sothat reliability of the transistor is lowered. For this reason, oxygenvacancies contained in the oxide semiconductor layer need to be reduced.The oxide layers in which oxygen vacancies are less likely to begenerated than in the oxide semiconductor layer 404 b are provided overand under and in contact with the oxide semiconductor layer 404 b in theoxide stack 404, whereby oxygen vacancies in the oxide semiconductorlayer 404 b can be reduced. For example, in the oxide semiconductorlayer 404 b, the absorption coefficient due to the localized states,which is obtained by measurement by a constant photocurrent method(CPM), is set lower than 1×10⁻³/cm, preferably lower than 1×10⁻⁴/cm.

Further, in the case where the oxide semiconductor layer 404 b is incontact with an insulating layer including a different constituentelement (e.g., a base insulating layer including a silicon oxide film),an impurity element (e.g., silicon) might be mixed into the oxidesemiconductor layer 404 b where a channel is formed. When an interfacestate is formed at an interface between the oxide semiconductor layer404 b and the insulating layer by the mixed impurity element, a decreasein electrical characteristics of the transistor, such as a change in athreshold voltage of the transistor, is caused. However, since the firstoxide layer 404 a contains one or more kinds of metal elements formingthe oxide semiconductor layer 404 b in the oxide stack 404, an interfacestate is not easily formed at an interface between the first oxide layer404 a and the oxide semiconductor layer 404 b. Thus, providing the firstoxide layer 404 a makes it possible to reduce fluctuation in theelectrical characteristics of the transistor, such as a thresholdvoltage.

In the case where a channel is formed in the vicinity of an interfacebetween the gate insulating layer 410 and the oxide semiconductor layer404 b, interface scattering occurs at the interface and the field-effectmobility of the transistor is decreased. However, since the second oxidelayer 404 c contains one or more kinds of metal elements forming theoxide semiconductor layer 404 b in the oxide stack 404, scattering ofcarriers does not easily occur at an interface between the second oxidelayer 404 c and the oxide semiconductor layer 404 b, and thus thefield-effect mobility of the transistor can be increased.

Further, the first oxide layer 404 a and the second oxide layer 404 ceach also serve as a barrier layer which suppresses formation of animpurity level due to the entry of the constituent elements of theinsulating layers which are in contact with the oxide stack 404 (theinsulating layer 402 and the gate insulating layer 410) into the oxidesemiconductor layer 404 b.

For example, in the case of using a silicon-containing insulating layeras each of the insulating layer 402 and the gate insulating layer 410which are in contact with the oxide stack 404, the silicon in theinsulating layers or carbon which might be contained in the insulatinglayers enters the first oxide layer 404 a or the second oxide layer 404c at a depth of several nanometers from the interface in some cases. Animpurity such as silicon or carbon entering the oxide semiconductorlayer forms impurity levels. The impurity levels serve as a donor andgenerate an electron, so that the oxide semiconductor layer may becomen-type.

However, when the thickness of the first oxide layer 404 a and thesecond oxide layer 404 c is larger than several nanometers, the impuritysuch as silicon or carbon does not reach the oxide semiconductor layer404 b, so that the influence of impurity levels is suppressed.

Here, the concentration of silicon in the oxide semiconductor layer islower than or equal to 3×10¹⁸ atoms/cm³, preferably lower than or equalto 3×10¹⁷ atoms/cm³. In addition, the concentration of carbon in theoxide semiconductor layer is lower than or equal to 3×10¹⁸ atoms/cm³,preferably lower than or equal to 3×10¹⁷ atoms/cm³. It is particularlypreferable to sandwich or surround the oxide semiconductor layer 404 bserving as a carrier path by the first oxide layer 404 a and the secondoxide layer 404 c in order to prevent entry of much silicon or carbon,which is a Group 14 element, to the oxide semiconductor layer 404 b.That is, the concentration of silicon and carbon contained in the oxidesemiconductor layer 404 b is preferably lower than that in the firstoxide layer 404 a and the second oxide layer 404 c.

Note that the impurity concentration of the oxide semiconductor layercan be measured by secondary ion mass spectrometry (SIMS).

If hydrogen or moisture is contained in the oxide semiconductor layer,it can work as a donor and the oxide semiconductor layer may becomen-type; therefore, in terms of achieving a well-shaped structure, it isuseful to provide a protective insulating layer (e.g., a silicon nitridelayer) for preventing entry of hydrogen or moisture from the outside,above the oxide stack 404.

Note that although trap levels due to impurities or defects might beformed in the vicinity of the interface between an insulating film suchas a silicon oxide film and each of the first oxide layer 404 a and thesecond oxide layer 404 c as shown in FIG. 14, the oxide semiconductorlayer 404 b can be distanced away from the trap levels owing to theexistence of the first oxide layer 404 a and the second oxide layer 404c. However, when the energy difference between EcS1 and EcS2 and theenergy difference between EcS2 and EcS3 are small, an electron in theoxide semiconductor layer 404 b might reach the trap level by passingover the energy difference. Since the electron is trapped in the traplevel, a negative fixed charge is caused at the interface with theinsulating film; thus, the threshold voltage of the transistor isshifted in the positive direction.

Thus, the energy difference between EcS1 and EcS2 and the energydifference between EcS3 and EcS2 are each preferably greater than orequal to 0.1 eV, further preferably greater than or equal to 0.15 eVbecause the amount of change of the threshold voltage of the transistoris reduced and the transistor has stable electrical characteristics.

[Formation of Oxide Stack]

Each of the oxide layers included in the multi-layer structure is formedusing a sputtering target which contains at least indium (In) and withwhich a film can be formed by a sputtering method, preferably a DCsputtering method. When the sputtering target contains indium, theconductivity thereof is increased; therefore, film formation by a DCsputtering method is facilitated.

As a material forming the first oxide layer 404 a and the second oxidelayer 404 c, a material which is represented by an In-M—Zn oxide (M is ametal element such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf) is used.As M, Ga is preferably used. However, a material with a high proportionof Ga, specifically the material represented as InGa_(x)Zn_(y)O_(z) withX exceeding 10, is not suitable because powder may be generated in thedeposition and deposition by a sputtering method may become difficult.

Note that for each of the first oxide layer 404 a and the second oxidelayer 404 c, a material in which the proportion of indium in the atomicratio is smaller than a material used for the oxide semiconductor layer404 b is used. The indium and gallium contents in the oxide layers 404 aand 404 c and the oxide semiconductor layer 404 b can be compared witheach other by time-of-flight secondary ion mass spectrometry (alsoreferred to as TOF-SIMS) or X-ray photoelectron spectrometry (alsoreferred to as XPS).

Each of the oxide layers included in the multi-layer structure may be ina non-single-crystal state, for example. The non-single-crystal stateis, for example, structured by at least one of c-axis aligned crystal(CAAC), polycrystal, microcrystal, and an amorphous part. Among c-axisaligned crystal (CAAC), polycrystal, microcrystal, and an amorphouspart, the amorphous part has the highest density of defect states,whereas CAAC has the lowest density of defect states.

Each of the oxide layers included in the multi-layer structure mayinclude microcrystal, for example. A microcrystalline oxide film, forexample, includes a microcrystal of greater than or equal to 1 nm andless than 10 nm.

Each of the oxide layers included in the multi-layer structure mayinclude an amorphous part, for example. An amorphous oxide film, forexample, has disordered atomic arrangement and no crystalline component.Alternatively, an amorphous oxide film is, for example, absolutelyamorphous and has no crystal part.

Note that the oxide layers included in the multi-layer structure mayeach be a mixed film including any of a CAAC oxide, a microcrystallineoxide, and an amorphous oxide. The mixed film, for example, includes aregion of an amorphous oxide, a region of a microcrystalline oxide, anda region of a CAAC oxide. Further, the mixed film may have a layeredstructure including a region of an amorphous oxide, a region of amicrocrystalline oxide, and a region of a CAAC oxide, for example.

Each of the oxide layers included in the multi-layer structure may be ina single-crystal state, for example.

It is preferable that each of the oxide layers included in themulti-layer structure include a plurality of crystal parts, and in eachof the crystal parts, a c-axis be aligned in a direction parallel to anormal vector of a surface where the oxide film is formed or a normalvector of a surface of the oxide film. Note that among crystal parts,the directions of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. An example of such anoxide semiconductor film is a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) film.

When the first oxide layer 404 a contains a constituent element (e.g.,silicon) of the insulating layer 402 as an impurity, it might have anamorphous structure. Note that the oxide semiconductor layer 404 b inwhich a channel is formed preferably has a crystal part. In the casewhere the oxide semiconductor layer 404 b having a crystal part isstacked over the first oxide layer 404 a having an amorphous structure,the oxide stack can be referred to as a hetero structure havingdifferent crystal structures.

In addition, the second oxide layer 404 c may have an amorphousstructure or include a crystal part. Formation of the second oxide layer404 c over the oxide semiconductor layer 404 b having a crystal partallows the second oxide layer 404 c to have a crystal structure. In thiscase, a boundary between the oxide semiconductor layer 404 b and thesecond oxide layer 404 c cannot be clearly identified by observation ofthe cross section with a transmission electron microscope (TEM) in somecases. Note that the second oxide layer 404 c has lower crystallinitythan the oxide semiconductor layer 404 b. Hence, it can be said that theboundary can be determined by the degree of crystallinity.

At least the oxide semiconductor layer 404 b in the oxide stack 404 ispreferably a CAAC-OS film. In this specification and the like, a CAAC-OSfilm refers to an oxide semiconductor film which includes a crystal partwhose c-axis is aligned in a direction substantially perpendicular tothe surface of the oxide semiconductor film.

A structure of an oxide semiconductor film is described below.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

An oxide semiconductor film is roughly classified into anon-single-crystal oxide semiconductor film and a single-crystal oxidesemiconductor film. The non-single-crystal oxide semiconductor filmincludes any of a CAAC-OS film, a polycrystalline oxide semiconductorfilm, a microcrystalline oxide semiconductor film, an amorphous oxidesemiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary does not easily occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface of the CAAC-OS film. Thus, for example, in thecase where a shape of the CAAC-OS film is changed by etching or thelike, the c-axis might not be necessarily parallel to a normal vector ofa formation surface or a normal vector of a top surface of the CAAC-OSfilm.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the CAAC-OS film occurs from the vicinity of the topsurface of the film, the proportion of the c-axis aligned crystal partsin the vicinity of the top surface is higher than that in the vicinityof the formation surface in some cases. Further, when an impurity isadded to the CAAC-OS film, a region to which the impurity is added isaltered, and the proportion of the c-axis aligned crystal parts in theCAAC-OS film varies depending on regions, in some cases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having a low impurityconcentration. The impurity is any of elements which are not the maincomponents of the oxide semiconductor film and includes hydrogen,carbon, silicon, a transition metal element, and the like. Inparticular, an element (e.g., silicon) which has higher bonding strengthwith oxygen than a metal element included in the oxide semiconductorfilm causes disorder of atomic arrangement in the oxide semiconductorfilm because the element deprives the oxide semiconductor film ofoxygen, thereby reducing crystallinity. Further, a heavy metal such asiron or nickel, argon, carbon dioxide, and the like have a large atomicradius (or molecular radius); therefore, when any of such elements iscontained in the oxide semiconductor film, the element causes disorderof the atomic arrangement of the oxide semiconductor film, therebyreducing crystallinity. Note that the impurity contained in the oxidesemiconductor film might become a carrier trap or a source of carriers.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. For example, oxygen vacancies in the oxide semiconductorfilm serve as carrier traps or serve as carrier generation sources whenhydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (few oxygen vacancies) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has few carrier generation sources, and thus has alow carrier density. Thus, a transistor using the oxide semiconductorfilm rarely has a negative threshold voltage (rarely has normally-oncharacteristics). A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier traps.Accordingly, the transistor using the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Note that charges trapped by the carrier traps in the oxidesemiconductor film take a long time to be released and may behave likefixed charges. Thus, the transistor using the oxide semiconductor filmwith a high impurity concentration and a high density of defect stateshas unstable electrical characteristics in some cases.

In a transistor using the CAAC-OS film, change in electricalcharacteristics due to irradiation with visible light or ultravioletlight is small.

Next, a microcrystalline oxide semiconductor film is described.

In an image obtained with a TEM, for example, crystal parts cannot befound clearly in the microcrystalline oxide semiconductor film in somecases. In most cases, the size of a crystal part included in themicrocrystalline oxide semiconductor film is greater than or equal to 1nm and less than or equal to 100 nm, or greater than or equal to 1 nmand less than or equal to 10 nm, for example. A microcrystal with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, or a sizegreater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as nanocrystal (nc). An oxide semiconductorfilm including nanocrystal is referred to as an nc-OS (nanocrystallineoxide semiconductor) film. In an image of the nc-OS film obtained with aTEM, for example, a boundary between crystal parts is not clearlydetected in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. However, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo pattern is shown in anelectron diffraction pattern (also referred to as a selected-areaelectron diffraction pattern) of the nc-OS film obtained by using anelectron beam having a probe diameter (e.g., larger than or equal to 50nm) larger than a diameter of a crystal part. Meanwhile, spots are shownin a nanobeam electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter (e.g., larger than orequal to 1 nm and smaller than or equal to 30 nm) close to, or smallerthan or equal to a diameter of a crystal part. Further, in a nanobeamelectron diffraction pattern of the nc-OS film, regions with highluminance in a circular (ring) pattern are shown in some cases. Also ina nanobeam electron diffraction pattern of the nc-OS film, a pluralityof spots are shown in a ring-like region in some cases.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more layers of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

The above is the description of the structure of the oxide semiconductorfilm.

Note that the first oxide layer 404 a and the second oxide layer 404 cwhich are included in the oxide stack 404 might have the same structureas the above oxide semiconductor layer.

Note that it is preferable that, in the oxide stack 404, the first oxidelayer 404 a have an amorphous structure and that a CAAC-OS film bedeposited from a surface of the amorphous structure to be used as theoxide semiconductor layer 404 b.

[Formation Method of the CAAC-OS Film]

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be formed.

The flat-plate-like sputtered particle has, for example, an equivalentcircle diameter of a plane parallel to the a-b plane of greater than orequal to 3 nm and less than or equal to 10 nm, and a thickness (lengthin the direction perpendicular to the a-b plane) of greater than orequal to 0.7 nm and less than 1 nm. Note that in the flat-plate-likesputtered particle, the plane parallel to the a-b plane may be a regulartriangle or a regular hexagon. Here, the term “equivalent circlediameter of a plane” refers to the diameter of a perfect circle havingthe same area as the plane.

For the formation of the CAAC-OS film, the following conditions arepreferably used.

By increasing the substrate temperature during the deposition, migrationof a sputtered particle easily occurs after the sputtered particlereaches a substrate surface. Specifically, the substrate temperatureduring the deposition is higher than or equal to 100° C. and lower thanor equal to 740° C., preferably higher than or equal to 200° C. andlower than or equal to 500° C. By increasing the substrate temperatureduring the deposition, when the flat-plate-like sputtered particlereaches the substrate, migration occurs on the substrate, so that a flatplane of the sputtered particle is attached to the substrate. At thistime, the sputtered particle is charged positively, whereby sputteredparticles are attached to the substrate while repelling each other;thus, the sputtered particles do not overlap with each other randomly,and a CAAC-OS film with a uniform thickness can be deposited.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, and nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

After the CAAC-OS film is deposited, heat treatment may be performed.The temperature of the heat treatment is higher than or equal to 100° C.and lower than or equal to 740° C., preferably higher than or equal to200° C. and lower than or equal to 500° C. Further, the heat treatmentis performed for 1 minute to 24 hours, preferably 6 minutes to 4 hours.The heat treatment may be performed in an inert atmosphere or anoxidation atmosphere. It is preferable to perform heat treatment in aninert atmosphere and then to perform heat treatment in an oxidationatmosphere. The heat treatment in an inert atmosphere can reduce theconcentration of impurities in the CAAC-OS film in a short time. At thesame time, the heat treatment in an inert atmosphere might generateoxygen vacancies in the CAAC-OS film. In this case, the heat treatmentin an oxidation atmosphere can reduce the oxygen vacancies. The heattreatment can further increase the crystallinity of the CAAC-OS film.Note that the heat treatment may be performed under a reduced pressure,such as 1000 Pa or lower, 100 Pa or lower, 10 Pa or lower, or 1 Pa orlower. The heat treatment under the reduced pressure can reduce theconcentration of impurities in the CAAC-OS film in a shorter time.

As an example of the sputtering target, an In—Ga—Zn oxide target isdescribed below.

The polycrystalline In—Ga—Zn oxide target is made by mixing InO_(X)powder, GaO_(Y) powder, and ZnO_(Z) powder at a predetermined molarratio, applying pressure, and performing heat treatment at a temperaturehigher than or equal to 1000° C. and lower than or equal to 1500° C.Note that X, Y, and Z are each a given positive number. Here, thepredetermined molar ratio of InO_(X) powder to GaO_(Y) powder andZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or3:1:2. The kinds of powder and the molar ratio for mixing powder may bedetermined as appropriate depending on the desired sputtering target.

Alternatively, the CAAC-OS film is formed by the following method.

First, a first oxide semiconductor film is formed to a thickness ofgreater than or equal to 1 nm and less than 10 nm. The first oxidesemiconductor film is formed by a sputtering method. Specifically, thesubstrate temperature is higher than or equal to 100° C. and lower thanor equal to 500° C., preferably higher than or equal to 150° C. andlower than or equal to 450° C., and the proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

Next, heat treatment is performed so that the first oxide semiconductorfilm serves as a first CAAC-OS film with high crystallinity. Thetemperature of the heat treatment is higher than or equal to 350° C. andlower than or equal to 740° C., preferably higher than or equal to 450°C. and lower than or equal to 650° C. Further, the heat treatment isperformed for 1 minute to 24 hours, preferably 6 minutes to 4 hours. Theheat treatment may be performed in an inert atmosphere or an oxidationatmosphere. It is preferable to perform heat treatment in an inertatmosphere and then to perform heat treatment in an oxidationatmosphere. The heat treatment in an inert atmosphere can reduce theconcentration of impurities in the first oxide semiconductor film in ashort time. At the same time, the heat treatment in an inert atmospheremight generate oxygen vacancies in the first oxide semiconductor film.In this case, the heat treatment in an oxidation atmosphere can reducethe oxygen vacancies. Note that the heat treatment may be performedunder a reduced pressure, such as 1000 Pa or lower, 100 Pa or lower, 10Pa or lower, or 1 Pa or lower. The heat treatment under the reducedpressure can reduce the concentration of impurities in the first oxidesemiconductor film in a shorter time.

The first oxide semiconductor film with a thickness of greater than orequal to 1 nm and less than 10 nm can be easily crystallized by heattreatment compared to the case where the first oxide semiconductor filmhas a thickness of greater than or equal to 10 nm.

Next, a second oxide semiconductor film that has the same composition asthe first oxide semiconductor film is formed to a thickness of greaterthan or equal to 10 nm and less than or equal to 50 nm. The second oxidesemiconductor film is formed by a sputtering method. Specifically, thesubstrate temperature is higher than or equal to 100° C. and lower thanor equal to 500° C., preferably higher than or equal to 150° C. andlower than or equal to 450° C., and the proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

Next, heat treatment is performed so that solid phase growth of thesecond oxide semiconductor film from the first CAAC-OS film occurs,whereby the second oxide semiconductor film is turned into a secondCAAC-OS film having high crystallinity. The temperature of the heattreatment is higher than or equal to 350° C. and lower than or equal to740° C., preferably higher than or equal to 450° C. and lower than orequal to 650° C. Further, the heat treatment is performed for 1 minuteto 24 hours, preferably 6 minutes to 4 hours. The heat treatment may beperformed in an inert atmosphere or an oxidation atmosphere. It ispreferable to perform heat treatment in an inert atmosphere and then toperform heat treatment in an oxidation atmosphere. The heat treatment inan inert atmosphere can reduce the concentration of impurities in thesecond oxide semiconductor film in a short time. At the same time, theheat treatment in an inert atmosphere might generate oxygen vacancies inthe second oxide semiconductor film. In this case, the heat treatment inan oxidation atmosphere can reduce the oxygen vacancies. Note that theheat treatment may be performed under a reduced pressure, such as 1000Pa or lower, 100 Pa or lower, 10 Pa or lower, or 1 Pa or lower. The heattreatment under a reduced pressure can reduce the concentration ofimpurities in the second oxide semiconductor film in a shorter time.

As described above, the CAAC-OS film with a total thickness of 10 nm ormore can be formed. The CAAC-OS film can be favorably used as the oxidesemiconductor layer in the oxide stack.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

(Embodiment 2)

In this embodiment, an example of a semiconductor device including thestacked structure described in Embodiment 1 and an example of a methodfor fabricating the semiconductor device will be described withreference to drawings. In this embodiment, a top gate transistorincluding an oxide semiconductor layer will be described as an exampleof the semiconductor device.

[Structural Example of Semiconductor Device]

FIGS. 1A to 1C illustrate a structural example of a transistor 100. FIG.1A is a schematic top view of the transistor 100, FIG. 1B is a schematiccross-sectional view taken along line A-B in FIG. 1A, and FIG. 1C is aschematic cross-sectional view taken along line C-D in FIG. 1A.

The transistor 100 illustrated in FIGS. 1A to 1C includes an insulatinglayer 103 provided over a substrate 101; a first oxide layer 104 aprovided over the insulating layer so as to have an island-like shape;an island-like oxide semiconductor layer 104 b provided over the firstoxide layer 104 a; an oxide layer 106 provided in contact with sidesurfaces of the first oxide layer 104 a and the oxide semiconductorlayer 104 b; an insulating layer 107 provided so as to surround the sidesurfaces of the first oxide layer 104 a and the oxide semiconductorlayer 104 b and to be in contact with a side surface of the oxide layer106; a pair of electrode layers 108 a provided in contact with part of atop surface of the insulating layer 107, part of a top surface of theoxide layer 106, and part of a top surface of the oxide semiconductorlayer 104 b; a pair of electrode layers 108 b provided over theelectrode layers 108 a and in contact with part of the top surface ofthe oxide semiconductor layer 104 b; a pair of electrode layers 108 cprovided over and in contact with the electrode layers 108 b; a secondoxide layer 104 c provided in contact with part of top surfaces of thepair of electrode layers 108 c and part of the top surface of the oxidesemiconductor layer 104 b; a gate insulating layer 105 provided over thesecond oxide layer 104 c; and an electrode layer 109 a and an electrodelayer 109 b stacked over the gate insulating layer 105. An insulatinglayer 111 is provided so as to cover the above-described structure.Further, an insulating layer 112 may be provided over the insulatinglayer 111.

A stack including the first oxide layer 104 a, the oxide semiconductorlayer 104 b, and the second oxide layer 104 c is an oxide stack 104.Note that boundaries of these layers, which are sometimes unclear, areindicated by dashed lines in FIGS. 1B and 1C.

In the oxide stack 104, low-resistance regions 102 are formed in regionswhich overlap with the electrode layers 108 a and the electrode layers108 b. FIG. 1B shows the low-resistance regions 102 which are formed inthe oxide semiconductor layer 104 b. Note that boundaries of thelow-resistance regions 102 in the oxide stack 104, which are sometimesunclear, are indicated by dashed lines in FIG. 1B.

A stack including the electrode layer 108 a, the electrode layer 108 b,and the electrode layer 108 c is an electrode layer 108. The electrodelayer 108 functions as a source electrode or a drain electrode of thetransistor 100. Further, a stack including the electrode layer 109 a andthe electrode layer 109 b is an electrode layer 109. The electrode layer109 functions as a gate electrode of the transistor 100.

The components are described below.

[Substrate]

The substrate 101 is not limited to a simple supporting substrate, andmay be a substrate where a device such as a transistor is formed. Inthis case, at least one of the gate electrode, the source electrode, andthe drain electrode which are included in the transistor 100 may beelectrically connected to the above device.

[Insulating Layer]

The insulating layer 103 has a function of supplying oxygen to the oxidestack 104 as well as a function of preventing diffusion of an impurityfrom the substrate 101; thus, an insulating layer containing oxygen isused as the insulating layer 103. Note that in the case where thesubstrate 101 is a substrate where another device is formed as describedabove, the insulating layer 103 also has a function as an interlayerinsulating layer. In that case, the insulating layer 103 is preferablysubjected to planarization treatment such as chemical mechanicalpolishing (CMP) treatment so as to have a flat surface.

In the transistor 100 in this embodiment, the insulating layer 103containing oxygen is provided below the stack structure (the oxide stack104) including the oxide semiconductor layer. With such a structure,oxygen in the insulating layer 103 can be supplied to a channelformation region. The insulating layer 103 preferably has a regioncontaining oxygen in excess of the stoichiometric composition. When theinsulating layer 103 contains oxygen in excess of the stoichiometriccomposition, supply of oxygen to the channel formation region can bepromoted.

Note that in this specification and the like, excess oxygen means oxygenwhich can be transferred in an oxide semiconductor layer, an oxidelayer, silicon oxide, or silicon oxynitride, oxygen which exists inexcess of the intrinsic stoichiometric composition, or oxygen having afunction of filling Vo (oxygen vacancies) generated due to lack ofoxygen.

[Gate Insulating Layer]

Oxygen might also be supplied to the oxide stack 104 from the gateinsulating layer 105 provided over and in contact with the oxide stack104.

The gate insulating layer 105 can be formed using an insulating layercontaining one or more of aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.Further, the gate insulating layer 105 may be a stacked layer of any ofthe above materials.

Since oxygen is supplied from both a bottom side and an upper side ofthe oxide stack 104, oxygen vacancies which might be included in theoxide stack 104 can be reduced.

[Protective Insulating Layer]

As the insulating layer 112 provided over the electrode layer 108 andthe electrode layer 109 in the transistor 100, an insulating layerhaving a lower oxygen-transmitting property (or higher oxygen barrierproperty) than the gate insulating layer 105 is used. For example, aninsulating layer having a lower oxygen-transmitting property than thegate insulating layer 105 or the insulating layer 107 is used. Sincepart of the insulating layer 112 covers the gate insulating layer 105and the oxide stack 104, the insulating layer 112 formed using such amaterial having a low oxygen-transmitting property can inhibitdesorption of oxygen from the oxide stack 104. Accordingly, generationof oxygen vacancies in the channel formation region can be inhibited.For the insulating layer 112, silicon nitride, silicon nitride oxide,aluminum oxide, or the like can be used, for example.

Further, the insulating layer 111 containing oxygen is preferablyprovided under the insulating layer 112. With such a structure, oxygenin the insulating layer 111 can be supplied to the channel formationregion. For the insulating layer 111, a material similar to that of theinsulating layer 103 can be used, for example.

The insulating layer 112 having a low oxygen-transmitting propertycovers the insulating layer 111 containing oxygen, whereby oxygen whichis released from the insulating layer 111 or the like is prevented fromleaking to the outside. Thus, it is possible to supply oxygen to thechannel formation region more effectively.

Further, in an oxide semiconductor, hydrogen behaves as a supply sourceof carriers in addition to oxygen vacancies. When the oxidesemiconductor contains hydrogen, a donor is generated at a level closeto the conduction band (a shallow level), and thus the oxidesemiconductor has lower resistance (n-type conductivity). Accordingly,the concentration of hydrogen in the insulating layer 111 is preferablyreduced. Specifically, the concentration of hydrogen in the insulatinglayer 111 is preferably lower than 5×10¹⁹ atoms/cm³, further preferablylower than 5×10¹⁸ atoms/cm³.

[Oxide Stack]

The oxide stack 104 includes at least the oxide semiconductor layer 104b in which a channel is formed, the first oxide layer 104 a which isprovided between the oxide semiconductor layer 104 b and the insulatinglayer 103, and the second oxide layer 104 c which is provided betweenthe oxide semiconductor layer 104 b and the gate insulating layer 105.

Each of the first oxide layer 104 a and the second oxide layer 104 c isan oxide layer containing one or more kinds of metal elements formingthe oxide semiconductor layer 104 b. For the details of the oxide stack104, refer to Embodiment 1. Here, the first oxide layer 104 a, the oxidesemiconductor layer 104 b, and the second oxide layer 104 c which areincluded in the transistor 100 correspond to the first oxide layer 404a, the oxide semiconductor layer 404 b, and the second oxide layer 404 cin Embodiment 1, respectively.

In the oxide stack 104, the oxide layers in which oxygen vacancies areless likely to be generated than in the oxide semiconductor layer 104 bare provided over and under and in contact with the oxide semiconductorlayer 104 b where a channel is formed, whereby generation of oxygenvacancies in the channel formation region of the transistor can beinhibited.

Note that in order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, the concentration of silicon in the oxidesemiconductor layer, which is measured by SIMS, is set to be lower than1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, furtherpreferably lower than 3×10¹⁸ atoms/cm³, still further preferably lowerthan 1×10¹⁸ atoms/cm³. The concentration of hydrogen in the oxidesemiconductor layer is set to be lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁹ atoms/cm³, still furtherpreferably lower than or equal to 5×10¹⁸ atoms/cm³. The concentration ofnitrogen in the oxide semiconductor layer is set to be lower than 5×10¹⁹atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁸ atoms/cm³, still furtherpreferably lower than or equal to 5×10¹⁷ atoms/cm³.

In the case where the oxide semiconductor layer includes crystals, highconcentration of silicon or carbon might reduce the crystallinity of theoxide semiconductor layer. The crystallinity of the oxide semiconductorlayer can be prevented from decreasing when the concentration of siliconis lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,further preferably lower than 1×10¹⁸ atoms/cm³, and the concentration ofcarbon is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³.

A transistor in which a highly purified oxide semiconductor layer isused for a channel formation region as described above has an extremelylow off-state current. For example, the drain current at the time whenthe transistor including a highly purified oxide semiconductor layer isin an off-state at room temperature (approximately 25° C.) can be lessthan or equal to 1×10⁻¹⁸ A, preferably less than or equal to 1×10⁻²¹ A,and further preferably 1×10⁻²⁴ A; or at 85° C., less than or equal to1×10⁻¹⁵ A, preferably less than or equal to 1×10⁻¹⁸ A, furtherpreferably less than or equal to 1×10⁻²¹ A. An off state of a transistorrefers to a state where a gate voltage is much lower than a thresholdvoltage in an n-channel transistor. Specifically, the transistor is inan off state when the gate voltage is lower than the threshold voltageby 1 V or more, 2 V or more, or 3 V or more.

[Sidewall Oxide Layer]

The oxide layer 106 is provided in contact with at least the sidesurface of the oxide semiconductor layer 104 b. It is preferable thatthe oxide layer 106 be provided in contact with the side surface of thefirst oxide layer 104 a and the side surface of the oxide semiconductorlayer 104 b.

The oxide layer 106 is formed using an oxide which contains one or morekinds of metal elements forming the oxide semiconductor layer 104 b. Forexample, the material which can be used for the first oxide layer 104 aor the second oxide layer 104 c described above can be employed. When amaterial containing In and M (M is a metal such as Al, Ti, Ga, Ge, Y,Zr, Sn, La, Ce, or Hf) is used for the oxide layer 106, an oxide whichcontains M more than In at an atomic ratio is preferably used.Alternatively, an M oxide (e.g., gallium oxide) can be used.

The width of the oxide layer 106 is greater than or equal to 0.1 nm andless than 10 nm, preferably greater than or equal to 0.5 nm and lessthan 5 nm, further preferably greater than or equal to 1 nm and lessthan 3 nm.

The oxide layer 106 in which oxygen vacancies are not easily generatedis provided in contact with the side surfaces of the oxide semiconductorlayer 104 b and the first oxide layer 104 a, whereby desorption ofoxygen from the side surfaces of the oxide semiconductor layer 104 b andthe first oxide layer 104 a is inhibited; thus, generation of oxygenvacancies can be inhibited. As a result, a transistor which has improvedelectrical characteristics and high reliability can be provided.

In this manner, the oxide semiconductor layer 104 b in which the channelis formed is surrounded by the first oxide layer 104 a, the second oxidelayer 104 c, and the oxide layer 106 in each of which oxygen vacanciesare not easily generated, whereby oxygen vacancies which might exist inthe channel can be reduced.

The insulating layer 107 is provided so as to surround the side surfaceof the oxide layer 106. In other words, the insulating layer 107 isprovided so as to surround the oxide semiconductor layer 104 b. Here, itis preferable that the top surface of the insulating layer 107 beplanarized by a planarization treatment. At this time, the level of thehighest region of the top surface of the oxide semiconductor layer 104 bis preferably lower than the level of the top surface of the insulatinglayer 107. It is further preferable that the level of the highest regionof the top surface of the insulating layer 107 be equal to the level ofthe highest region of the top surface of the oxide semiconductor layer104 b. The level of the top surface of the oxide layer 106 may be equalto the level of the top surface of the insulating layer 107 or the levelof the highest region of the top surface of the oxide semiconductorlayer 104 b. Alternatively, the level of the top surface of the oxidelayer 106 may be higher than the level of the highest region of the topsurface of the oxide semiconductor layer 104 b and lower than or equalto the level of the top surface of the insulating layer 107.

Here, the levels of top surfaces of two layers are determined bydistances from a planar surface which is located below the two layers.For example, it is possible to use a distance from a top surface of thesubstrate 101 or a distance from a top surface of the insulating layer103 which is planarized.

As described above, the side surface of the oxide semiconductor layer104 b is surrounded by the insulating layer 107 and the top surface ofthe oxide semiconductor layer 104 b is level with or located below theinsulating layer 107. In other words, the oxide semiconductor layer 104b is embedded in the insulating layer 107. Such a structure can also becalled a shallow trench structure.

Here, it is preferable that a side surface of an end portion of a stackincluding the first oxide layer 104 a and the oxide semiconductor layer104 b be substantially perpendicular to a surface where the stack isformed (e.g., the surface of the insulating layer 103), as illustratedin FIGS. 1B and 1C. When the side surface is processed in that way, thearea occupied by the stack including the first oxide layer 104 a and theoxide semiconductor layer 104 b can be reduced, so that higherintegration can be achieved.

Note that the end portion of the stack including the first oxide layer104 a and the oxide semiconductor layer 104 b is tapered in some casesas illustrated in FIG. 2A, depending on processing conditions for thefirst oxide layer 104 a and the oxide semiconductor layer 104 b. Whenthe end portion of the stack including the first oxide layer 104 a andthe oxide semiconductor layer 104 b is processed to be tapered, coveragewith a layer (e.g., the insulating layer 107) which is provided over thestack can be improved.

The above-described shallow trench structure in one embodiment of thepresent invention has various advantages as described below.

Owing to the oxide semiconductor layer 104 b (and the first oxide layer104 a) embedded in the insulating layer 107, the above structure doesnot include a step at an end portion which exists in the case of formingthe oxide semiconductor layer 104 b (and the first oxide layer 104 a) asa thin film(s) over a planar surface. Thus, at the time when theelectrode layer 108 and the electrode layer 109 are formed, it is notnecessary to take into account coverage at a portion which extendsbeyond the step, so that the degree of freedom of the process can beincreased. In addition, since a thin region is not formed at the endportion of the oxide semiconductor layer 104 b (and the first oxidelayer 104 a), the thickness of the oxide semiconductor layer 104 b in aregion in which the electrode layer 109 overlaps with the end portioncan be uniform; therefore, the transistor can have favorable electricalcharacteristics.

Further, at the time when a plurality of the stacks of the oxidesemiconductor layer 104 b and the first oxide layer 104 a are providedadjacent to each other, distances between the stacks can be small ascompared to the case of formation which uses a thin film over a planarsurface. Thus, it can be said that the transistor of one embodiment ofthe present invention can be highly integrated.

[Source Electrode and Drain Electrode]

In the transistor 100, the electrode layer 108, which functions as thesource electrode or the drain electrode, includes the electrode layer108 a, the electrode layer 108 b, and the electrode layer 108 c. Theelectrode layer 108 a is provided in contact with the top surfaces ofthe insulating layer 107, the oxide layer 106, and the oxidesemiconductor layer 104 b. A stack including the electrode layer 108 band the electrode layer 108 c is provided so as to extend beyond an endportion of the electrode layer 108 a on the channel side and to be incontact with the top surface of the oxide semiconductor layer 104 b.

For the electrode layer 108 a and the electrode layer 108 b, aconductive material which is easily bonded to oxygen can be used. Forexample, Al, Cr, Cu, Ta, Ti, Mo, or W can be used. W or Ti with arelatively high melting point is preferably used, which allowssubsequent process temperatures to be relatively high. Note that theconductive material which is easily bonded to oxygen includes, in itscategory, a material to which oxygen is easily diffused.

When such a conductive material which is easily bonded to oxygen is incontact with the oxide stack 104, oxygen in the oxide stack 104 is takeninto the conductive material. There are some heating steps in afabrication process of the transistor, and thus the above transfer ofoxygen causes generation of oxygen vacancies in a region of the oxidestack 104 which is in the vicinity of an interface between the oxidestack 104 and the electrode layer 108 a or 108 b, so that an n-typeregion (the low-resistance region 102) is formed. Accordingly, thelow-resistance region 102 can serve as a source or a drain of thetransistor 100.

The low-resistance region 102 is formed in a region of the oxide stack104 which is in contact with the electrode layer 108 a or 108 b asdescribed above, whereby contact resistance between the electrode layer108 a or 108 b and the oxide semiconductor layer 104 b is reduced andparasitic resistance between a source and a drain of the transistor 100can be reduced; accordingly, the transistor 100 can have favorableelectrical characteristics.

Note that a constituent element of the electrode layer 108 a or 108 bmight enter the low-resistance region 102. Further, part of theelectrode layer 108 a or 108 b, which is in contact with thelow-resistance region 102, might have a region having high oxygenconcentration. A constituent element of the oxide stack 104 might enterthe part of the electrode layer 108 a or 108 b, which is in contact withthe low-resistance region 102. That is, in the vicinity of the interfacebetween the oxide stack 104 and the electrode layer 108 a or 108 b, aportion which can also be called a mixed region or a mixed layer of theoxide stack 104 and the electrode layer 108 a or 108 b is formed in somecases. Note that in FIG. 1B, an interface between the low-resistanceregion 102 and the electrode layer 108 a or 108 b is schematicallyindicated by a dotted line.

A conductive material which is not easily bonded to oxygen is used forthe electrode layer 108 c. As the conductive material, for example, ametal nitride such as tantalum nitride or titanium nitride is preferablyused. The electrode layer 108 c which is not easily bonded to oxygen isprovided in contact with a top surface of the electrode layer 108 b,whereby oxygen diffused from the oxide stack 104 can be prevented fromdiffusing to above the electrode layer 108 b through the electrode layer108 b in a region of the electrode layer 108 b which is in contact withthe oxide semiconductor layer 104 b, and thus it is possible to preventtoo much oxygen from being extracted from the oxide stack 104. Note thatthe conductive material which is not easily bonded to oxygen includes,in its category, a material to which oxygen is not easily diffused.

At this time, by control of the thickness of the electrode layer 108 b,the depth of the low-resistance region 102 formed in a region of theoxide stack 104 which is in contact with the electrode layer 108 b canbe controlled. For example, when the electrode layer 108 b is formed tobe thinner than the electrode layer 108 a, the low-resistance region 102can have a smaller depth in a region overlapping with the electrodelayer 108 b than in a region overlapping with the electrode layer 108 aas illustrated in FIG. 1B.

The low-resistance region 102 with a smaller depth in the vicinity ofthe channel formation region can function as a lightly doped drain (LDD)region and can inhibit degradation of the characteristics of thetransistor 100.

Note that in the low-resistance region 102, the conductivity may varycontinuously in the depth direction. For example, the shallower regionoften has lower resistance. In addition, the region overlapping with theelectrode layer 108 a and the region overlapping with the electrodelayer 108 b may have different conductivities. In that case, in thelow-resistance region 102, a shallow region formed in the vicinity ofthe channel formation region preferably has lower conductivity than adeeper region, in which case degradation of the characteristics of thetransistor 100 can be more inhibited.

To control the depth of the low-resistance region 102, any of themethods below can also be employed.

In one method, the materials used for the electrode layer 108 a and theelectrode layer 108 b are different in the degree of ease of beingbonded to oxygen. For example, when W is used for the electrode layer108 a and Ti is used for the electrode layer 108 b, the depth of thelow-resistance region 102 can differ between the region which is incontact with the electrode layer 108 a and the region which is incontact with the electrode layer 108 b.

In another method, as illustrated in FIG. 2B, an electrode layer 108 din which the degree of ease of being bonded to oxygen is controlled isused instead of the stack including the electrode layer 108 b and theelectrode layer 108 c. For the electrode layer 108 d, a metal nitride inwhich the amount of added nitrogen is adjusted to be relatively small (ametal nitride film whose nitrogen concentration is reduced) can be used.In the case of using titanium nitride, for example, it is possible touse a material in which the proportion of titanium and the proportion ofnitrogen in the composition satisfy the following relation: 0<N<Ti.

FIG. 21 shows compositions of titanium nitride films which were formedwith deposition gases having various flow rate ratios. The films wereeach formed over a glass substrate by a sputtering method. Thedeposition conditions were as follows: the pressure was 0.2 Pa, a DCpower of 12 kW was used, a distance between a substrate and a target was400 mm, and a substrate temperature at the time of deposition was roomtemperature. A mixed gas of nitrogen and argon was used as thedeposition gas, and the films were deposited with the proportion ofnitrogen changed from 0% to 100%. The proportions of nitrogen andtitanium in the composition of the deposited films were measured byRutherford backscattering spectrometry (RBS). As shown in FIG. 21, asthe proportion of nitrogen in the deposition gas is increased, thenitrogen content in the film can be increased. In the titanium nitridefilm, the nitrogen content tends to be saturated when the proportion ofnitrogen in the deposition gas is about 70%, and the proportion ofnitrogen in the composition of the titanium nitride film formed usingthe deposition gas in which the proportion of nitrogen is about 70% wasabout 54%.

In a region overlapping with the electrode layer 108 d in which thedegree of ease of being bonded to oxygen is controlled, the depth of thelow-resistance region 102 formed in the oxide stack 104 can becontrolled.

FIGS. 3A to 3C are enlarged schematic diagrams of the vicinity of thelow-resistance region 102 in FIG. 1B. Here, the low-resistance region102 in the oxide stack 104 may be formed in only the oxide semiconductorlayer 104 b as illustrated in FIG. 3A. Alternatively, as illustrated inFIG. 3B, the low-resistance region 102 may be formed to reach thevicinity of an interface between the oxide semiconductor layer 104 b andthe first oxide layer 104 a in the depth direction. Furtheralternatively, as illustrated in FIG. 3C, the low-resistance region 102reaches the first oxide layer 104 a in some cases.

Note that in the transistor having the structure illustrated in FIGS. 1Ato 1C, the channel length refers to a distance between the pair ofelectrode layers 108 b.

Further, in the transistor having the structure illustrated in FIGS. 1Ato 1C, a channel is formed in a part of the oxide semiconductor layer104 b which is between the pair of electrode layers 108 b.

Furthermore, in the transistor having the structure illustrated in FIGS.1A to 1C, a channel formation region means parts of the first oxidelayer 104 a, the oxide semiconductor layer 104 b, and the second oxidelayer 104 c which are between the pair of electrode layers 108 b.

[Gate Electrode Layer]

The electrode layer 109 has a structure in which the electrode layer 109a and the electrode layer 109 b are stacked. Note that the electrodelayer 109 may be a stack including three or more conductive layers.

For the electrode layer 109 b, it is possible to use a conductive filmof a metal material such as Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag,Ta, or W, or an alloy material containing the above metal material.

For the electrode layer 109 a, which is provided on the oxidesemiconductor layer 104 b side, the above-described conductive materialwhich is not easily bonded to oxygen can be used. When the electrodelayer 109 a which is not easily bonded to oxygen is provided between theelectrode layer 109 b and the gate insulating layer 105, oxygen in theoxide stack 104 can be prevented from diffusing to the electrode layer109 b through the gate insulating layer 105, which leads to highreliability of the transistor.

For example, a titanium nitride film and a titanium-tungsten alloy filmare used for the electrode layer 109 a and the electrode layer 109 b,respectively. Alternatively, a tantalum nitride film and a tungsten filmare used for the electrode layer 109 a and the electrode layer 109 b,respectively. Further alternatively, a structure can be employed inwhich a titanium nitride film and a tungsten film are used for theelectrode layer 109 a and the electrode layer 109 b, respectively, and atitanium-tungsten alloy film is included between the electrode layer 109a and the electrode layer 109 b.

Note that when oxygen is not easily diffused from the gate insulatinglayer 105, the electrode layer 109 a is not necessarily provided.

Although the electrode layers which are described in the aboveembodiments can be formed by a sputtering method, such layers may beformed by another method, e.g., a thermal CVD method. A metal organicchemical vapor deposition (MOCVD) method or an atomic layer deposition(ALD) method may be employed as an example of a thermal CVD method.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

The above is the descriptions of the components.

FIG. 4 is an enlarged schematic diagram illustrating the channelformation region of the transistor 100.

It is preferable that as illustrated in FIG. 4, the oxide semiconductorlayer 104 b be partly etched such that the top surface of the oxidesemiconductor layer 104 b in the channel formation region is locatedbelow the depth of the low-resistance region 102 formed in the vicinityof the channel formation region. In addition, the second oxide layer 104c is preferably formed in contact with a top surface of a thin region ofthe oxide semiconductor layer 104 b.

Here, in the oxide semiconductor layer 104 b, a region whose resistanceis reduced due to extraction of oxygen by the electrode layer 108 bextends not only in the depth direction but also in the channel lengthdirection in some cases. Therefore, when the channel length is setextremely small, the pair of low-resistance regions 102 might beconnected to each other to be short-circuited.

However, such short circuit between the pair of low-resistance regions102 can be prevented when the top surface of the oxide semiconductorlayer 104 b is etched to be located below the depth of thelow-resistance region 102 in the channel formation region as illustratedin FIG. 4.

In FIG. 4, a dashed arrow schematically illustrates a path of a currentwhich mainly flows between the pair of electrode layers 108. The channelis mainly formed in the oxide semiconductor layer 104 b, so that acurrent also flows mainly in the oxide semiconductor layer 104 b. Thelarger a difference between the level of the bottom surface of thelow-resistance region 102 in the vicinity of the channel formationregion and the level of the top surface of the oxide semiconductor layer104 b is, the longer the channel length can be; thus, a short channeleffect can be inhibited. Even a transistor in which an actual channellength is extremely short can have favorable electrical characteristics.

The channel length of the transistor of one embodiment of the presentinvention can be set to as short as 30 nm or less, preferably 20 nm orless, further preferably 10+X nm (X is greater than or equal to 0 andless than 10) or less.

MODIFICATION EXAMPLE

In the formation process of the transistor 100, a capacitor can also beformed without an increase of the number of steps of the process.

In FIGS. 5A and 5B, a structural example is illustrated in which acapacitor 150 is formed so as to be electrically connected to thetransistor 100.

The capacitor 150 illustrated in FIG. 5A has a structure in which partof the electrode layer 108 a, an electrode layer 158 b, an electrodelayer 158 c, an oxide layer 154, an insulating layer 155, an electrodelayer 159 a, and an electrode layer 159 b are stacked in order.

The electrode layer 158 b can be formed by processing the same film asthe electrode layer 108 b. Similarly, the electrode layer 158 c, theoxide layer 154, the insulating layer 155, the electrode layer 159 a,and the electrode layer 159 b can be formed by processing the same filmsas the electrode layer 108 c, the second oxide layer 104 c, the gateinsulating layer 105, the electrode layer 109 a, and the electrode layer109 b, respectively. Thus, the capacitor 150 can be formed at the sametime as fabrication of the transistor 100 without an increase of thenumber of steps.

The capacitor 150 illustrated in FIG. 5B has a structure in which partof the electrode layer 108 a, the oxide layer 154, the insulating layer155, the electrode layer 159 a, and the electrode layer 159 b arestacked in order.

In each of the above structures, a stack including the oxide layer 154and the insulating layer 155 functions as a dielectric of the capacitor.

Here, when an oxide semiconductor is used for the oxide layer 154, arelative permittivity higher than a relative permittivity of aninsulator such as silicon oxide can be achieved. For example, whilesilicon oxide has a relative permittivity of 4.0 to 4.5, an oxidesemiconductor can have a relative permittivity of 13 to 17 or 14 to 16.Therefore, without a reduction in capacitance, the thickness of theoxide layer 154 can be large and thus a withstand voltage of thecapacitor can be increased.

Further, as illustrated in FIGS. 5A and 5B, the capacitor can be formedover the insulating layer 107 outside the region (also referred to astrench region) in which the oxide semiconductor layer 104 b is embedded.

With such a structure, the transistor 100 and the capacitor 150 can befabricated at the same time without an increase of the number of steps.Therefore, a semiconductor circuit including the transistor 100 and thecapacitor 150 can be easily fabricated.

The above is the description of this modification example.

[Example of Fabrication Method of Transistor]

An example of a fabrication method of the transistor described above isdescribed below with reference to the drawings.

First, the insulating layer 103 is formed over the substrate 101.

The insulating layer 103 can be formed by a plasma chemical vapordeposition (CVD) method, a sputtering method, or the like using aninsulating film containing oxygen, such as an insulating film ofaluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.

It is preferable that the insulating layer 103 be formed with a materialcontaining oxygen, which can supply the oxygen to at least the oxidestack 104 to be formed later. Further, the insulating layer 103 ispreferably a film excessively containing oxygen.

In order to make the insulating layer 103 excessively contain oxygen,the insulating layer 103 may be formed in an oxygen atmosphere, forexample. Alternatively, the insulating layer 103 may excessively containoxygen in such a manner that oxygen is introduced into the insulatinglayer 103 which has been formed. Both the methods may be combined.

For example, oxygen (at least including any of oxygen radicals, oxygenatoms, and oxygen ions) is introduced into the insulating layer 103which has been formed, whereby a region excessively containing oxygen isformed. As a method for introducing oxygen, an ion implantation method,an ion doping method, a plasma immersion ion implantation method, plasmatreatment, or the like can be used.

A gas containing oxygen can be used for oxygen introducing treatment. Asthe gas containing oxygen, oxygen, dinitrogen monoxide, nitrogendioxide, carbon dioxide, carbon monoxide, and the like can be used.Further, a rare gas may be contained in the gas containing oxygen in theoxygen introducing treatment.

Then, the first oxide layer 104 a and the oxide semiconductor layer 104b are formed over the insulating layer 103 by a sputtering method, a CVDmethod, a molecular beam epitaxy (MBE) method, an atomic layerdeposition (ALD) method, or a pulsed laser deposition (PLD) method (seeFIG. 6A).

For the first oxide layer 104 a, the oxide semiconductor layer 104 b,and the second oxide layer 104 c which is to be formed later, thematerial described in Embodiment 1 as an example can be used.

For example, the first oxide layer 104 a is preferably formed using anIn—Ga—Zn oxide having an atomic ratio of In to Ga and Zn which is 1:3:2,an In—Ga—Zn oxide having an atomic ratio of In to Ga and Zn which is1:6:4, an In—Ga—Zn oxide having an atomic ratio of In to Ga and Zn whichis 1:9:6, or an oxide having a composition which is in the neighborhoodof any of the above atomic ratios.

Further, for example, the oxide semiconductor layer 104 b is preferablyformed using an In—Ga—Zn oxide having an atomic ratio of In to Ga and Znwhich is 1:1:1, an In—Ga—Zn oxide having an atomic ratio of In to Ga andZn which is 3:1:2, or an oxide having a composition which is in theneighborhood of any of the above atomic ratios.

For example, the second oxide layer 104 c is preferably formed using anIn—Ga—Zn oxide having an atomic ratio of In to Ga and Zn which is 1:3:2,an In—Ga—Zn oxide having an atomic ratio of In to Ga and Zn which is1:6:4, an In—Ga—Zn oxide having an atomic ratio of In to Ga and Zn whichis 1:9:6, or an oxide having a composition which is in the neighborhoodof any of the above atomic ratios.

Note that for example, the expression “the composition of an oxidecontaining In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example.

Note that the composition of each of the layers is not limited to theabove atomic ratios. The indium content in the oxide semiconductor layer104 b is preferably higher than that in the first oxide layer 104 a andthat in the second oxide layer 104 c. In an oxide semiconductor, the sorbital of heavy metal mainly contributes to carrier transfer, and whenthe proportion of In in the oxide semiconductor is increased, overlap ofthe s orbitals is likely to be increased. Therefore, an oxide having acomposition in which the proportion of In is higher than that of Ga hashigher mobility than an oxide having a composition in which theproportion of In is equal to or lower than that of Ga. Further, galliumneeds large formation energy of an oxygen vacancy and thus is not likelyto generate an oxygen vacancy as compared to indium. Therefore, an oxidehaving a high gallium content has stable characteristics.

Thus, with the use of an oxide having a high indium content for theoxide semiconductor layer 104 b, a transistor having high mobility canbe achieved. Further, when an oxide having a high gallium content isused for on a side of an interface with each of the insulating layers,the transistor can have higher reliability.

Further, an oxide semiconductor that can be used for the first oxidelayer 104 a, the oxide semiconductor layer 104 b, and the second oxidelayer 104 c preferably contains at least indium (In) or zinc (Zn).Alternatively, the oxide semiconductor preferably contains both In andZn. Specifically, it is preferable that the oxide semiconductor layer104 b contain indium because the carrier mobility of the transistor canbe increased, and the oxide semiconductor layer 104 b contain zincbecause a CAAC-OS film is formed easily. In order to reduce fluctuationin electrical characteristics of the transistors including the oxidesemiconductor layer, the oxide semiconductor preferably contains astabilizer in addition to indium and zinc.

As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),zirconium (Zr), and the like can be given. As another stabilizer,lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu) can be given.

For example, any of the following oxides, or specifically oxidesemiconductors, can be used: indium oxide, tin oxide, zinc oxide, anIn—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mgoxide, an In—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Znoxide, an In—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, aSn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide,an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Znoxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, anIn—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide,an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, anIn—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and anIn—Hf—Al—Zn oxide.

Note that as described in Embodiment 1 in detail, a material of thefirst oxide layer 104 a and the second oxide layer 104 c is selected sothat the first oxide layer 104 a and the second oxide layer 104 c havehigher electron affinities than that of the oxide semiconductor layer104 b.

Note that the oxide stack is preferably formed by a sputtering method.As a sputtering method, an RF sputtering method, a DC sputtering method,an AC sputtering method, or the like can be used. In particular, a DCsputtering method is preferably used because dust generated in thedeposition can be reduced and the film thickness can be uniform.

Note that oxygen may be introduced into the first oxide layer 104 a attiming which is after formation of the first oxide layer 104 a andbefore formation of the oxide semiconductor layer 104 b. By the oxygenintroduction treatment, the first oxide layer 104 a excessively containsoxygen, so that the excess oxygen can be supplied to the oxidesemiconductor layer 104 b by heat treatment in a later film formationstep.

The preferable and typical amount of oxygen which is added to the firstoxide layer 104 a is, in the case of using an ion implantation method, adosage higher than or equal to 5×10¹⁴/cm² and lower than or equal to1×10¹⁷/cm². It is preferable that the amount of oxygen which is added belarge enough to enable a reduction in oxygen vacancies in the oxidesemiconductor film which is to be formed later, and the amount istypically 5×10¹⁴/cm² or more, preferably 1×10¹⁵/cm² or more. At the sametime, the higher the amount of added oxygen, the longer the treatmenttime and the lower the productivity; thus, the amount is preferably1×10¹⁷/cm² or less, further preferably 5×10¹⁶/cm² or less, still furtherpreferably 2×10¹⁶/cm² or less.

Accordingly, oxygen vacancies in the oxide semiconductor layer 104 b canbe suppressed more by the oxygen introduction treatment performed on thefirst oxide layer 104 a.

Note that the crystallinity of the first oxide layer 104 a is lowered bythe oxygen introduction treatment in some cases. In the oxide stack 104,at least the oxide semiconductor layer 104 b is preferably a CAAC-OSfilm. Accordingly, the oxygen introduction treatment is preferablyperformed at timing which is after formation of the first oxide layer104 a and before formation of the oxide semiconductor layer 104 b.

Then, the stack including the first oxide layer 104 a and the oxidesemiconductor layer 104 b is selectively etched, so that an island-likestack including the first oxide layer 104 a and the oxide semiconductorlayer 104 b is formed. Note that heating may be performed beforeetching.

Subsequently, the oxide layer 106 is formed so as to be in contact withat least a side surface of the stack including the first oxide layer 104a and the oxide semiconductor layer 104 b (FIG. 6B). The oxide layer 106is formed by a sputtering method, a CVD method, an MBE method, an ALDmethod, or a PLD method.

When a material containing In and M (M is a metal such as Al, Ti, Ga,Ge, Y, Zr, Sn, La, Ce, or Hf) is used for the oxide layer 106, an oxidewhich contains M more than In at an atomic ratio is preferably used.

For example, the oxide layer 106 is preferably formed using an In—Ga—Znoxide having an atomic ratio of In to Ga and Zn which is 1:3:2, anIn—Ga—Zn oxide having an atomic ratio of In to Ga and Zn which is 1:6:4,an In—Ga—Zn oxide having an atomic ratio of In to Ga and Zn which is1:9:6, or an oxide having a composition which is in the neighborhood ofany of the above atomic ratios. A film of such an oxide is preferablyformed by a sputtering method.

Alternatively, an oxide (e.g., gallium oxide) not containing indium canbe used for the oxide layer 106. In this case, a film of the oxide ispreferably formed by a CVD method.

Next, the oxide layer 106 except for that in a region in contact withthe side surfaces of the first oxide layer 104 a and the oxidesemiconductor layer 104 b is removed by etching. For example, byanisotropic etching using a dry etching method or the like, only theoxide layer 106 in the region in contact with the side surfaces of thefirst oxide layer 104 a and the oxide semiconductor layer 104 b can beleft. In this manner, the oxide layer 106 in contact with the sidesurfaces of the first oxide layer 104 a and the oxide semiconductorlayer 104 b can be formed.

Then, the insulating layer 107 is formed so as to cover the oxidesemiconductor layer 104 b and the oxide layer 106 (FIG. 6C). Theinsulating layer 107 can be formed by a plasma CVD method, a sputteringmethod, or the like using an insulating film of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or thelike.

The insulating layer 107 is then subjected to planarization treatment,so that the top surface of the oxide semiconductor layer 104 b isexposed (FIG. 6D). To the planarization treatment, a CMP method or thelike can be applied.

In some cases, the planarization treatment causes a reduction in thethickness of the oxide semiconductor layer 104 b. In that case, with athickness reduced by the planarization treatment taken intoconsideration, the oxide semiconductor layer 104 b having a largethickness is preferably formed in advance.

After the planarization treatment, heat treatment is preferablyperformed. Owing to the heat treatment, efficient oxygen supply can beperformed from the insulating layer 103 to the first oxide layer 104 aand the oxide semiconductor layer 104 b and from the first oxide layer104 a to the oxide semiconductor layer 104 b, whereby oxygen vacanciesin the first oxide layer 104 a and the oxide semiconductor layer 104 bcan be reduced. Further, by the heat treatment, the crystallinity of thefirst oxide layer 104 a and the oxide semiconductor layer 104 b can beincreased, and moreover, an impurity such as hydrogen or water can beremoved from at least one of the insulating layer 107, the first oxidelayer 104 a, the oxide semiconductor layer 104 b, and the oxide layer106.

The heat treatment may be performed at a temperature higher than orequal to 250° C. and lower than or equal to 650° C., preferably higherthan or equal to 300° C. and lower than or equal to 500° C., in an inertgas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm ormore, or a reduced pressure state. Alternatively, the heat treatment maybe performed in such a manner that heat treatment is performed in aninert gas atmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more in order tocompensate desorbed oxygen.

FIG. 8 is a schematic perspective view of this stage. As illustrated inthe drawing, the side surface of the island-like stack including thefirst oxide layer 104 a and the oxide semiconductor layer 104 b iscovered with the oxide layer 106, and the island-like stack is embeddedin the insulating layer 107. The top surfaces of the oxide semiconductorlayer 104 b, the oxide layer 106, and the insulating layer 107 areplanarized. The levels of the top surfaces are substantially the same,or a difference between the levels is extremely small. It is thuspossible to prevent an adverse effect due to a step at the time offormation of a layer over these layers. When a plurality of the aboveisland-like stacks are provided, a distance between adjacent stacks canbe made extremely small.

Then, a conductive film is formed over the oxide semiconductor layer 104b, the oxide layer 106, and the insulating layer 107 and selectivelyetched so as to be divided over the oxide semiconductor layer 104 b;thus, the pair of electrode layers 108 a are formed.

For the electrode layers 108 a, Al, Cr, Cu, Ta, Ti, Mo, W, or an alloymaterial containing any of these as a main component can be used, forexample. For example, a tungsten film is formed by a deposition methodsuch as a sputtering method.

At this time, the end portion of the electrode layer 108 a is preferablyformed so as to have a staircase-like shape as illustrated in thedrawing. The end portion can be formed in such a manner that a step ofmaking a resist mask recede by ashing and an etching step arealternately performed a plurality of times. By making the end portionhave a staircase-like shape, coverage with a layer (e.g., the electrodelayer 108 b or the electrode layer 108 c) to be formed thereover can beimproved, and the layer to be formed can be thus thin. In addition, theelectrode layer 108 a can be formed thick, whereby the resistance of theelectrode can be reduced.

Note that although not shown, by overetching of the conductive film,part of the oxide semiconductor layer 104 b or the insulating layer 107(an exposed region) is etched in some cases.

Next, over the oxide semiconductor layer 104 b, the electrode layers 108a, and the insulating layer 107, a conductive film to be the electrodelayers 108 b and a conductive film to be the electrode layers 108 c areformed and selectively etched so as to be divided over the oxidesemiconductor layer 104 b; thus, a pair of stacks of the electrodelayers 108 b and the electrode layers 108 c are formed (FIG. 6E).

For the electrode layers 108 b, Al, Cr, Cu, Ta, Ti, Mo, W, or an alloymaterial containing any of these as a main component can be used, forexample. For the electrode layers 108 c, it is possible to use a film ofmetal nitride such as tantalum nitride or titanium nitride, or a nitridefilm of an alloy material containing tantalum, titanium, or the like asa main component. For example, a titanium film is formed by a sputteringmethod and a titanium nitride film is then formed by a sputteringmethod.

When the electrode layers 108 b and the electrode layers 108 c areformed by processing the conductive films by etching, they arepreferably overetched so that part of the oxide semiconductor layer 104b is etched intentionally. At this time, the oxide semiconductor layer104 b is preferably etched such that the top surface of the oxidesemiconductor layer 104 b is located below the depth of thelow-resistance region 102 to be formed later in the vicinity of thechannel formation region.

Note that with a reduction in the thickness of the oxide semiconductorlayer 104 b due to processing of the conductive film into the electrodelayers 108 a, the electrode layers 108 b, and the electrode layers 108 ctaken into consideration, the oxide semiconductor layer 104 b having alarge thickness is preferably formed in advance.

Note that in the case of forming a transistor whose channel length isextremely short, at least a region to divide the stack including theconductive films to be the electrode layers 108 b and the electrodelayers 108 c is etched using resist masks that are processed by a methodsuitable for fine line processing, such as electron beam exposure. Notethat by the use of a positive type resist for the resist masks, theexposed region can be minimized and throughput can be thus improved. Inthe above manner, a transistor having a channel length of 30 nm or lesscan be formed. Alternatively, minute processing may be performed by anexposure technology which uses light with an extremely short wavelength(e.g., extreme ultraviolet (EUV)), X-rays, or the like.

Then, the second oxide layer 104 c is formed over the oxidesemiconductor layer 104 b, the electrode layers 108 a, the electrodelayers 108 c, and the insulating layer 107. The second oxide layer 104 cis formed by a sputtering method, a CVD method, an MBE method, an ALDmethod, or a PLD method using the above-described material.

Here, oxygen is preferably introduced into the second oxide layer 104 cat timing which is after formation of the second oxide layer 104 c. Bythe oxygen introduction treatment, the second oxide layer 104 cexcessively contains oxygen, so that the excess oxygen can be suppliedto the oxide semiconductor layer 104 b by heat treatment in a later filmformation step.

The preferable and typical amount of oxygen which is added to the secondoxide layer 104 c is, in the case of using an ion implantation method, adosage higher than or equal to 5×10¹⁴/cm² and lower than or equal to1×10¹⁷/cm². It is preferable that the amount of oxygen which is added belarge enough to enable a reduction in oxygen vacancies in the oxidesemiconductor film which is to be formed later, and the amount istypically 5×10¹⁴/cm² or more, preferably 1×10¹⁵/cm² or more. At the sametime, the higher the amount of added oxygen, the longer the treatmenttime and the lower the productivity; thus, the amount is preferably1×10¹⁷/cm² or less, further preferably 5×10¹⁶/cm² or less, still furtherpreferably 2×10¹⁶/cm² or less.

Next, heat treatment is performed, so that the low-resistance regions102 are formed (FIG. 7A). When the heat treatment is performed in astate where the electrode layers 108 a are in contact with the oxidesemiconductor layer 104 b, oxygen in the stack including the oxidesemiconductor layer 104 b and the first oxide layer 104 a is taken intothe electrode layers 108 a which are easily bonded to oxygen.Accordingly, oxygen vacancies are generated in regions of the oxidesemiconductor layer 104 b which are in the vicinities of the interfaceswith the electrode layers 108 a, so that the low-resistance regions 102are formed. Similarly, by the heat treatment, the low-resistance regions102 are formed in regions of the oxide semiconductor layer 104 b whichare in the vicinities of the interfaces with the electrode layers 108 b.

Here, in accordance with the thickness, material, and the like of theelectrode layer 108 a and the electrode layer 108 b, the depth of thelow-resistance region 102, which is formed thereunder, is determined.The depth can also be controlled in accordance with conditions of theheat treatment (temperature, time, pressure, or the like). For example,the higher the heating temperature or the longer the heating time, thelarger the depth of the low-resistance region 102. Note that thelow-resistance region 102 is not formed in some cases depending on thetemperature of the heat treatment.

The heat treatment may be performed at a temperature higher than orequal to 250° C. and lower than or equal to 650° C., preferably higherthan or equal to 300° C. and lower than or equal to 500° C., in an inertgas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm ormore, or a reduced pressure state. Alternatively, the heat treatment maybe performed in such a manner that heat treatment is performed in aninert gas atmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more in order tocompensate desorbed oxygen.

By the heat treatment, efficient oxygen supply can be performed from thesecond oxide layer 104 c to the oxide semiconductor layer 104 b, wherebyoxygen vacancies in the oxide semiconductor layer 104 b can be reduced.Further, by the heat treatment, the crystallinity of the first oxidelayer 104 a, the oxide semiconductor layer 104 b, and the second oxidelayer 104 c can be increased, and moreover, an impurity such as hydrogenor water can be removed from at least one of the insulating layer 107,the first oxide layer 104 a, the oxide semiconductor layer 104 b, thesecond oxide layer 104 c, and the oxide layer 106.

Next, the gate insulating layer 105 is formed over the second oxidelayer 104 c. The gate insulating layer 105 can be formed using aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, or the like. Note that the gate insulating layer 105 maybe a stacked layer of any of the above materials. The gate insulatinglayer 105 can be formed by a sputtering method, a CVD method, an MBEmethod, an ALD method, a PLD method, or the like. In particular, it ispreferable that the gate insulating layer 105 be formed by a CVD method,further preferably a plasma CVD method, because favorable coverage canbe achieved.

After the formation of the gate insulating layer 105, heat treatment ispreferably performed. By the heat treatment, an impurity such as wateror hydrogen contained in the gate insulating layer 105 can be desorbed(dehydration or dehydrogenation can be performed). The temperature ofthe heat treatment is preferably higher than or equal to 300° C. andlower than or equal to 400° C. The heat treatment may be performed insuch a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more in order tocompensate desorbed oxygen. By the heat treatment, an impurity such ashydrogen or water can be removed from the gate insulating layer 105. Inaddition, impurities such as hydrogen and water are further removed fromthe first oxide layer 104 a, the oxide semiconductor layer 104 b, andthe second oxide layer 104 c in some cases. Further, when the heattreatment is performed in an atmosphere containing an oxidizing gas,oxygen can be supplied to the gate insulating layer 105.

Note that it is preferable that the heat treatment be successivelyperformed in a deposition chamber after the gate insulating layer 105 isformed. Alternatively, the heating at the time of forming the gateinsulating layer 105 can serve as the heat treatment.

Then, a conductive film to be the electrode layer 109 a and a conductivefilm to be the electrode layer 109 b are formed in order. The conductivefilms can be formed by a sputtering method or the like.

Next, the conductive film to be the electrode layer 109 b, theconductive film to be the electrode layer 109 a, the gate insulatinglayer 105, and the second oxide layer 104 c are selectively etched so asto overlap with the channel formation region, so that a stack includingthe second oxide layer 104 c, the gate insulating layer 105, theelectrode layer 109 a, and the electrode layer 109 b is formed (FIG.7B).

Here, in the above etching step, the electrode layers 108 b and theelectrode layers 108 c can serve as etching stoppers.

Note that heat treatment may be performed after the conductive film tobe the electrode layer 109 a and the conductive film to be the electrodelayer 109 b are formed, or after the above etching step. To the heattreatment, the conditions of the heat treatment which can be performedafter the formation of the gate insulating layer 105 can be applied.

Here, the heat treatment for the formation of the low-resistance regions102 is not necessarily performed just after the second oxide layer 104 cis formed and may be performed at any timing after the second oxidelayer 104 c is formed. The low-resistance regions 102 can be formed byperforming heat treatment at least once. When heat treatment isperformed a plurality of times, oxygen vacancies in the oxide stack 104can be reduced more effectively.

Then, the insulating layer 111 and the insulating layer 112 are formedin order over the insulating layer 107, the electrode layers 108, andthe electrode layer 109 (FIG. 7C).

It is preferable that the insulating layer 111 be formed with a materialcontaining oxygen, which can supply the oxygen to the oxide stack 104.Further, the insulating layer 111 is preferably a film containing excessoxygen. To the formation of the insulating layer 111, a material and amethod similar to those of the insulating layer 103 can be applied.

Further, the insulating layer 112 is preferably formed using aninsulating film with a low oxygen-transmitting property (or with anoxygen barrier property). For example, a nitride such as silicon nitrideor silicon nitride oxide is preferably used. The insulating layer can beformed by a method such as a plasma CVD method or a sputtering method.It is preferable that the insulating layer 112 be formed by a sputteringmethod because the concentration of hydrogen in the insulating layer 112is preferably reduced. The concentration of hydrogen in the insulatinglayer 112 is preferably lower than 5×10¹⁹ atoms/cm³, further preferablylower than 5×10¹⁸ atoms/cm³.

Further, heat treatment may be performed after the formation of theinsulating layer 112. For example, the heat treatment is performed underconditions of the heat treatment which can be performed after theformation of the gate insulating layer 105, whereby oxygen can besupplied from the insulating layer 111 to the channel formation region.

In this manner, the transistor 100 in this embodiment can be fabricated.

The above is the description of this fabrication method.

Modification Example 1

An example which is partly different from the above fabrication methodof the transistor is described below. Specifically, in this modificationexample, steps up to and including the formation step of the insulatinglayer 107 in the above-described example of the fabrication method ofthe transistor are described.

First, in a manner similar to the above, the insulating layer 103, thefirst oxide layer 104 a, and the oxide semiconductor layer 104 b areformed over the substrate 101.

Over the oxide semiconductor layer 104 b, a resist mask 161 is formed(FIG. 9A). The resist mask 161 can be formed by photolithography.

Then, the oxide semiconductor layer 104 b in a region where the resistmask 161 is not provided is etched by a dry etching method, so that thefirst oxide layer 104 a is exposed.

After that, the first oxide layer 104 a which is exposed is etched by adry etching method. At this time, a reaction product of the first oxidelayer 104 a is attached to side surfaces of the first oxide layer 104 a,the oxide semiconductor layer 104 b, and the resist mask 161 to form anoxide layer 166 serving as a sidewall protective layer (also referred toas a sidewall oxide layer or a rabbit ear) (FIG. 9B). Note that thereaction product of the first oxide layer 104 a is attached by asputtering phenomenon or through plasma at the time of the dry etching.The dry etching may be performed under the following conditions, forexample: a boron trichloride gas and a chlorine gas are used as etchinggases, and inductively coupled plasma (ICP) power and substrate biaspower are applied.

Since the oxide layer 166 is formed of the reaction product of the firstoxide layer 104 a, main components of the oxide layer 166 are the sameas components of the first oxide layer 104 a.

At this time, the insulating layer 103 might be also partly etched, inwhich case the oxide layer 166 contains components of the insulatinglayer 103 (e.g., silicon).

Note that since the oxide layer 166 is formed of the reaction product ofthe first oxide layer 104 a, components of the etching gas used at thetime of etching (e.g., chlorine and boron) remain therein in some cases.

Then, the resist mask 161 is removed.

Then, the insulating layer 107 is formed so as to cover the oxidesemiconductor layer 104 b and the oxide layer 166 (FIG. 9C). Theinsulating layer 107 may be formed in a manner similar to the above.

The insulating layer 107 is then subjected to planarization treatment,so that the top surface of the oxide semiconductor layer 104 b isexposed (FIG. 9D).

At this time, a part of the oxide layer 166 which protrudes above thetop surface of the oxide semiconductor layer 104 b is also subjected tothe planarization treatment, whereby the levels of top surfaces theinsulating layer 107, the oxide layer 166, and the oxide semiconductorlayer 104 b can be substantially the same.

In this manner, a structure can be provided which includes the oxidesemiconductor layer 104 b (and the first oxide layer 104 a) embedded inthe insulating layer 107, and the oxide layer 166 surrounding the sidesurfaces of the oxide semiconductor layer 104 b and the first oxidelayer 104 a.

The above-described fabrication method of the transistor is applied tothe subsequent steps; accordingly, a highly reliable transistor can befabricated.

The above is the description of this modification example.

In the fabrication method described in this modification example, it ispossible to skip steps of film formation and etching which are performedto form the oxide layer in contact with the side surfaces of the oxidesemiconductor layer 104 b and the first oxide layer 104 a, so that theprocess can be simplified.

Modification Example 2

An example of a fabrication method of a transistor which is partlydifferent from the above fabrication method is described below.Specifically, in this modification example, in a manner similar toModification Example 1, steps up to and including the formation step ofthe insulating layer 107 in the above-described example of the methodfor fabricating the transistor are described.

First, in a manner similar to the above, the insulating layer 103, thefirst oxide layer 104 a, and the oxide semiconductor layer 104 b areformed over the substrate 101.

A barrier layer 171 is formed over the oxide semiconductor layer 104 b(FIG. 10A).

The barrier layer 171 has a function of preventing the oxidesemiconductor layer 104 b from being etched by planarization treatmentto be performed later.

For the barrier layer 171, a material resistant to the planarizationtreatment is selected. Any of an insulator, a conductor, and asemiconductor can be used since the barrier layer 171 is removed laterby etching. For example, a film formed by a sputtering method or a CVDmethod using silicon nitride or aluminum oxide may be used.

A stack including the first oxide layer 104 a, the oxide semiconductorlayer 104 b, and the barrier layer 171 is selectively etched to beprocessed into an island-like shape.

Next, the oxide layer 106 is formed by a method similar to theabove-described method (FIG. 10B).

Then, the oxide layer 106 except for that in a region in contact withside surfaces of the first oxide layer 104 a, the oxide semiconductorlayer 104 b, and the barrier layer 171 is removed by anisotropicetching, so that the oxide layer 106 which is in contact with a sidesurface of the stack including the first oxide layer 104 a, the oxidesemiconductor layer 104 b, and the barrier layer 171 is formed.

After that, by a method similar to the above-described method, theinsulating layer 107 is formed so as to cover the oxide layer 106 andthe barrier layer 171 (FIG. 10C).

Then, the insulating layer 107 is subjected to planarization treatment,so that top surfaces of the barrier layer 171 and the oxide layer 106are exposed (FIG. 10D).

At this time, since the barrier layer 171 is provided over the oxidesemiconductor layer 104 b, a reduction in the thickness of the oxidesemiconductor layer 104 b due to the planarization treatment can beprevented. In addition, owing to the barrier layer 171, the degree offreedom in setting conditions of the planarization treatment can beincreased.

Then, the barrier layer 171 is removed by etching (FIG. 10E). At thetime of removing the barrier layer 171, the conditions are preferablyset such that at least the oxide semiconductor layer 104 b is not easilyetched.

In this manner, a structure can be provided which includes the oxidesemiconductor layer 104 b (and the first oxide layer 104 a) embedded inthe insulating layer 107, and the oxide layer 106 surrounding the sidesurfaces of the oxide semiconductor layer 104 b and the first oxidelayer 104 a.

Here, the top surface of the oxide semiconductor layer 104 b is locatedbelow the top surfaces of the oxide layer 106 and the insulating layer107 after the removal of the barrier layer 171, whereby a step is formedbetween the oxide semiconductor layer 104 b and the oxide layer 106 insome cases. Thus, in order to reduce an adverse effect on coverage witha layer to be provided over the oxide semiconductor layer 104 b and theoxide layer 106, the thickness of the barrier layer 171 is preferablysmall. The barrier layer 171 is preferably formed as thin as possible,as long as the barrier layer is resistant to the planarizationtreatment. The thickness may be greater than or equal to 0.1 nm and lessthan or equal to 10 nm, preferably greater than or equal to 1 nm andless than or equal to 5 nm, further preferably greater than or equal to1 nm and less than or equal to 3 nm. When the barrier layer 171 isformed thin, damage to the oxide semiconductor layer 104 b at the timeof etching of the barrier layer 171 can be reduced.

The above-described fabrication method of the transistor is applied tothe subsequent steps; accordingly, a highly reliable transistor can befabricated.

Note that the oxide layer in contact with the side surfaces of the oxidesemiconductor layer 104 b and the first oxide layer 104 a can also beformed by the method described in Modification Example 1. In that case,at the same time as processing of the first oxide layer 104 a, the oxidesemiconductor layer 104 b, and the barrier layer 171 into an island-likeshape, the oxide layer in contact with the side surfaces of the firstoxide layer 104 a, the oxide semiconductor layer 104 b, and the barrierlayer 171 may be formed.

The above is the description of this modification example.

By the fabrication method described in this modification example, areduction in the thickness of the oxide semiconductor layer 104 b due tothe planarization treatment can be inhibited. Further, the top surfaceof the oxide semiconductor layer 104 b is not directly processed byplanarization treatment; thus, physical, chemical, or thermal damage tothe oxide semiconductor layer 104 b can be reduced. Therefore, byapplication of such a method, a transistor with excellent electricalcharacteristics and improved reliability can be provided.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

(Embodiment 3)

In this embodiment, a semiconductor device of one embodiment of thepresent invention which includes the transistor described in the aboveembodiment will be described with reference to the drawing.

The semiconductor device in this embodiment includes a plurality oftransistors, including the transistor described in the above embodiment.At least one of the plurality of transistors is stacked in thelongitudinal direction in order to increase the degree of integration.

[Structural Example 1 of Semiconductor Device]

FIG. 15A is a circuit diagram of a semiconductor device 400 which isdescribed in this embodiment. The semiconductor device 400 includes thetransistor 100 and the capacitor 150 which are described in the aboveembodiment, and a transistor 401.

The connection relation in the semiconductor device 400 is as follows. Agate of the transistor 401 is electrically connected to one electrode ofthe capacitor 150 and the source or the drain of the transistor 100.

Another circuit element (e.g., a transistor or a capacitor) may beelectrically connected to any of a gate of the transistor 100, thesource or the drain of the transistor 100, the other electrode of thecapacitor 150, and a source and a drain of the transistor 401.

The transistor 100 is an n-channel transistor which includes an oxidesemiconductor, as described in the above embodiment.

The transistor 401 is an n-channel transistor which includes asemiconductor material other than an oxide semiconductor. For example, acompound semiconductor such as a silicon-based semiconductor, agermanium-based semiconductor, gallium arsenide, or gallium nitride canbe used. By the use of a single crystal substrate, a polycrystallinesubstrate, a silicon on insulator (SOI) substrate, or the like for thetransistor 401, a transistor which operates at high speed can be easilyfabricated.

The transistor 100 includes an oxide semiconductor, and at least in itschannel formation region, high purity is achieved by sufficient removalof impurities such as hydrogen and oxygen vacancies are reduced; thus,an off-state current (also referred to as leakage current or off leakagecurrent) of the transistor is reduced.

In addition, since an off-state current of the transistor 100 isextremely low, electric charge can be held between the source or thedrain of the transistor 100, one electrode of the capacitor 150, and thegate of the transistor 401. In other words, the semiconductor device 400can function as a semiconductor memory device.

In the semiconductor device 400, since the transistor 100 includes theoxide semiconductor, power consumption is small as compared to the casewhere the semiconductor device illustrated in the circuit diagram inFIG. 15A is all formed using a semiconductor material other than anoxide semiconductor.

FIG. 15B is a cross-sectional view illustrating a cross-sectionalstructure of the semiconductor device 400.

In the semiconductor device 400, the transistor 100 and the capacitor150 are stacked over the transistor 401 with interlayer insulatinglayers provided therebetween. Details of the semiconductor device 400are described below.

The transistor 401 can be formed using a substrate 403, which includes asemiconductor material. Here, a single crystal silicon substrate havingp-type conductivity is used, and the channel formation region of thetransistor 401 is formed in the substrate 403. Note that the substrate403 is not limited to a single crystal silicon substrate having p-typeconductivity and can be a single crystal silicon substrate having n-typeconductivity, an SOI substrate, a glass substrate on whichpolycrystalline silicon is formed, or the like.

The transistor 401 includes a channel formation region 405 which isprovided in the substrate 403; impurity regions 407 between which thechannel formation region 405 is sandwiched; high-concentration impurityregions 409 which are electrically connected to the impurity regions 407(the impurity regions 407 and the high-concentration impurity regions409 are also referred to as impurity regions collectively); a gateinsulating layer 411 which is provided over the channel formation region405; a gate electrode layer 413 which is provided over the gateinsulating layer 411; and sidewall insulating layers 415 which areprovided on side surfaces of the gate electrode layer 413.

An insulating layer 419 is provided over the transistor 401, and aninterlayer insulating layer 421 is provided over the insulating layer419. An opening reaching the high-concentration impurity region 409 isprovided in the insulating layer 419 and the interlayer insulating layer421. In the opening, a source electrode layer or a drain electrode layer(hereinafter referred to as an electrode layer 416) of the transistor401 is provided.

A wiring layer 423 is provided in contact with the electrode layer 416.The wiring layer 423 is provided in contact with the source electrodelayer or the drain electrode layer to function as a source wiring or adrain wiring. The wiring layer 423 is electrically connected to otherelements which are included in the semiconductor device 400, otherdevices, or the like.

In addition, element isolation insulating layers 417 are provided on thesubstrate 403 so as to surround the transistor 401. The insulating layer419 is provided so as to cover the transistor 401 and the elementisolation insulating layers 417.

The impurity region 407 functions as an LDD region or an extensionregion. The high-concentration impurity region 409 functions as a sourceregion or a drain region of the transistor 401.

An interlayer insulating layer 425 is provided over the interlayerinsulating layer 421. A wiring layer 427 is provided over the interlayerinsulating layer 425. The wiring layer 427 functions as a wiring. Thewiring layer 427 is electrically connected to a gate wiring (notillustrated) which is electrically connected to the gate electrode layer413 through an opening (not illustrated) which is formed in theinsulating layer 419, the interlayer insulating layer 421, and theinterlayer insulating layer 425. The gate wiring is provided over thegate insulating layer 411 and branches to be the gate electrode layer413.

An interlayer insulating layer 429 is provided over the interlayerinsulating layer 425 and the wiring layer 427. The transistor 100 andthe capacitor 150 are provided over the interlayer insulating layer 429.Note that for details of the transistor 100 and the capacitor 150, theabove embodiment can be referred to.

An electrode layer 431 is provided so as to penetrate the interlayerinsulating layer 429, the insulating layer 103, and the insulating layer107 and to be in contact with the wiring layer 427 and the electrodelayer 108 a of the transistor 100, which also functions as one electrodeof the capacitor 150.

Note that in the semiconductor device 400, an insulating layer 433 isprovided over the transistor 100 and an insulating layer 435 is providedover the insulating layer 433. An interlayer insulating layer 437 isprovided over the insulating layer 435. An opening reaching theelectrode layer 108 c of the transistor 100 is provided in theinsulating layer 433, the insulating layer 435, and the interlayerinsulating layer 437, and in the opening, an electrode layer 439 isprovided. A wiring layer 441 is provided in contact with the electrodelayer 439. At least the wiring layer 441 functions as a source wiring ora drain wiring of the transistor 100.

Here, a fabrication method of the transistor 401 is described.

The element isolation insulating layers 417 are formed over thesubstrate 403 that is the single crystal silicon substrate having p-typeconductivity. For example, the element isolation insulating layers 417are formed as follows: a protective layer is formed over the substrate403, and etching treatment is performed using the protective layer as amask to remove the substrate 403 in regions which are not covered withthe protective layer (exposed regions), whereby recessed portions areformed. For the etching treatment, dry etching is preferably performed,but wet etching may be performed. An etching gas and an etchant can beselected as appropriate depending on a material to be etched. Then, aninsulating film is formed so as to cover the substrate 403 and theinsulating film which are formed in regions except the recessed portionsare selectively removed, whereby the element isolation insulating layers417 can be formed. The insulating film can be formed using a siliconoxide film, a silicon nitride film, a silicon nitride oxide film, or thelike. Polishing treatment such as chemical mechanical polishing (CMP),etching treatment, or the like can be employed as a way to remove theinsulating film. Note that the protective layer is removed after therecessed portions are formed or after the element isolation insulatinglayers 417 are formed. Further, it is preferable that after thepolishing treatment such as CMP or the etching treatment, cleaningtreatment be performed and heat treatment for removing moisture attachedonto a surface to be processed be performed.

Note that also in the fabrication process after the formation of theelement isolation insulating layers 417, the cleaning treatment and theheat treatment are preferably performed to remove moisture whenpolishing treatment such as CMP or etching treatment is performed.

Note that the element isolation insulating layers 417 can also be formedusing an element isolation technique such as local oxidation of silicon(LOCOS).

Note that a p-well may be formed by addition of an impurity elementimparting p-type conductivity to a region where the transistor 401 isformed. As the impurity element imparting p-type conductivity, boron(B), aluminum (Al), gallium (Ga), or the like can be used. Addition ofthe impurity element can be performed by an ion doping method, an ionimplantation method, or the like.

In the case of using a single crystal silicon substrate having n-typeconductivity as the substrate 403, an impurity element imparting p-typeconductivity may be added to form a p-well. In that case, the channelformation region 405 of the transistor 401 is formed in the p-well.

Then, an insulating film is formed over the substrate 403 where theelement isolation insulating layers 417 are formed; a conductive film isformed over the insulating film and processed to form the gate electrodelayer 413. With the use of the gate electrode layer 413 as a mask, theinsulating film is processed to form the gate insulating layer 411. Thegate insulating layer 411 can be formed using an insulating film and aformation method thereof that are applicable to the gate insulatinglayer 105 of the transistor 100 as appropriate. The gate electrode layer413 can be formed using a conductive film and a formation method thereofthat are applicable to the electrode layer 109 of the transistor 100 asappropriate.

Alternatively, the insulating film processed into the gate insulatinglayer 411 may be formed in such a manner that the surface of thesubstrate 403 is oxidized or nitrided by high-density plasma treatmentor thermal oxidation treatment. The high-density plasma treatment can beperformed using, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and a gas such as oxygen, nitrogen oxide, ammonia, nitrogen,or hydrogen.

Next, a protective layer is provided over the element isolationinsulating layers 417, and an impurity element imparting n-typeconductivity is added with the use of the protective layer and the gateelectrode layer 413 as masks, so that the impurity regions are formed.Note that by the formation of the impurity regions, a region of thesubstrate 403 which is located below the gate electrode layer 413 servesas the channel formation region 405 of the transistor 401 (see FIG.15B). The concentration of the added impurity can be set as appropriate;the concentration is preferably raised in accordance with the degree ofminiaturization of the semiconductor element. Alternatively, theimpurity element may be added through an insulating film (an insulatingfilm to be processed into the sidewall insulating layers 415) which isformed so as to cover the gate electrode layer 413. Note that theprotective layer is removed after the impurity element is added.

Next, the sidewall insulating layers 415 are formed. The insulating filmis formed so as to cover the gate electrode layer 413 and is thensubjected to highly anisotropic etching, whereby the sidewall insulatinglayers 415 can be formed in a self-aligned manner.

Next, an insulating film to be processed into the insulating layer 419is formed so as to cover the gate electrode layer 413, the impurityregions, the sidewall insulating layers 415, and the like. Then, aprotective layer is formed over the element isolation insulating layers417, and an impurity element imparting n-type conductivity is added tothe impurity regions using the protective layer, the gate electrodelayer 413, and the sidewall insulating layers 415 as masks, whereby theimpurity regions 407 and the high-concentration impurity regions 409 areformed. Note that the impurity element may be added before theinsulating film to be processed into the insulating layer 419 is formed,and then the insulating film may be formed. Note that the protectivelayer is removed after the impurity regions are formed.

Note that the semiconductor device that is one embodiment of the presentinvention is not limited to the semiconductor device 400 illustrated inFIG. 15B. For example, transistors including silicide (salicide) ortransistors without sidewalls may be used as the transistor 401. When astructure that contains silicide (salicide) is used, the resistance of asource region and a drain region can be further lowered and theoperating speed of the semiconductor device can be increased. Further,the semiconductor device can be operated at a low voltage; thus, powerconsumption of the semiconductor device can be reduced. Examples of ametal material with which silicide can be formed include titanium,tantalum, tungsten, nickel, cobalt, and platinum.

Then, an insulating film to be processed into the interlayer insulatinglayer 421 is formed over the insulating film to be processed into theinsulating layer 419, and these insulating films are processed, so thatopenings reaching the high-concentration impurity regions 409 are formedin the insulating layer 419 and the interlayer insulating layer 421.After that, a conductive film is formed in the openings and processed,whereby the electrode layers 416 are formed. The insulating layer 419,the interlayer insulating layer 421, and the openings can be formed bydry etching or the like, and the electrode layers 416 can be formed bypolishing treatment such as CMP. The electrode layers 416 are formed bythe polishing treatment or the like, whereby the insulating layer 419and the interlayer insulating layer 421 can be planarized. Note that theelectrode layers 416 may be formed by a combination of dry etching orthe like with polishing treatment.

For the interlayer insulating layer 421, an inorganic insulating film oran organic insulating film can be used. Examples of the inorganicinsulating film include a silicon oxide film, a silicon oxynitride film,an aluminum oxide film, and the like. Examples of the organic insulatingfilm include films of an acrylic resin, polyimide, abenzocyclobutene-based resin, polyamide, an epoxy resin, asiloxane-based resin, and the like. Note that the interlayer insulatinglayer 421 may be a stack including two or more of these insulatingfilms. There is no particular limitation on the method for forming theinorganic insulating film or the organic insulating film and the methodmay be selected as appropriate in accordance with the material to beused. For example, a CVD method, a sputtering method, spin coating,dipping, spray coating, a droplet discharge method (such as an inkjetmethod), screen printing, offset printing, or the like can be employed.

In the case where miniaturization of a semiconductor element such as atransistor included in a semiconductor device is advanced, parasiticcapacitance between wirings is significant and thus signal delay isincreased. In some cases, the parasitic capacitance cannot besufficiently inhibited with silicon oxide having a dielectric constantof 4.0 to 4.5; thus, a material with k of 3.0 or less is preferably usedfor forming the interlayer insulating layer 421. Further, mechanicalstrength is required for the interlayer insulating layer 421 becauseplanarization or the like is to be performed thereon. The interlayerinsulating layer 421 can be made porous to have a lower dielectricconstant as long as the mechanical strength can be secured.

It is preferable that the electrode layers 416 each have a structure inwhich part of the electrode layer is embedded in the interlayerinsulating layer 421 like a damascene structure. The electrode layers416 can be formed using a conductive film and a formation method thereofthat are applicable to the electrode layer 108 and the electrode layer109 of the transistor 100.

Note that in the case where the electrode layers 416 are formed,processing is preferably performed so that the surfaces thereof areplanarized. For example, when a thin titanium film or a thin titaniumnitride film is formed in a region including the openings and then atungsten film is formed so as to be embedded in the openings, an excesstungsten film, an excess titanium film, an excess titanium nitride film,or the like can be removed and the planarity of the surface of theelectrode layer 416 can be improved by the subsequent CMP.

Then, the wiring layers 423 are formed in contact with the electrodelayers 416, so that the interlayer insulating layer 425 is formed overthe wiring layers 423 and the interlayer insulating layer 421. Thewiring layers 423 can be formed in such a manner that a conductive filmapplicable to the electrode layers 416 is formed and processed by dryetching or the like. The interlayer insulating layer 425 can be formedin a manner similar to the interlayer insulating layer 421.

After that, the opening (not illustrated) reaching the gate electrodelayer 413 is formed in the insulating layer 419, the interlayerinsulating layer 421, and the interlayer insulating layer 425, and anelectrode layer (or the gate wiring) (not illustrated) is formed in theopening; then, the wiring layer 427 is formed over the interlayerinsulating layer 425 to be in contact with the electrode layer. Thesesteps can be performed in a manner similar to the formation steps of theopenings reaching the high-concentration impurity regions 409, theelectrode layers 416, and the wiring layers 423.

Next, an insulating film to be processed into the interlayer insulatinglayer 429 is formed over the interlayer insulating layer 425 and thewiring layer 427. For the insulating film, the insulating filmapplicable to the interlayer insulating layer 421 can be used.

Then, the steps up to and including the formation step of the insulatinglayer 107 of the transistor 100 are performed, and an opening reachingthe wiring layer 427 is formed in the interlayer insulating layer 429,the insulating layer 103, and the insulating layer 107. The electrodelayer 431 is formed in the opening. The above embodiment can be referredto for the steps up to and including the formation step of theinsulating layer 107 of the transistor 100. The steps before theformation of the electrode layer 431 can be performed in a mannersimilar to the formation steps of the openings reaching thehigh-concentration impurity regions 409, and the electrode layers 416.

Then, the steps subsequent to the formation of the insulating layer 107of the transistor 100 are performed, so that the transistor 100 and thecapacitor 150 are formed. For the steps, the above embodiment can bereferred to.

The insulating layer 433 is then formed over the transistor 100 and thecapacitor 150, and the insulating layer 435 is formed over theinsulating layer 433. The insulating layer 433 and the insulating layer435 can be formed using an insulating film and a formation methodthereof that are applicable to the insulating layer 103 and theinsulating layer 107 of the transistor 100. Note that since theinsulating layer 433 and the insulating layer 435 function as protectiveinsulating layers of the transistor 100, it is preferable to use aninsulating film which can inhibit entry of hydrogen or a hydride such aswater from the outside.

Then, the interlayer insulating layer 437 is formed over the insulatinglayer 435. The interlayer insulating layer 437 can be formed using aninsulating film and a formation method thereof which are applicable tothe interlayer insulating layer 421. The opening reaching the electrodelayer 108 c of the transistor 100 is formed in the insulating layer 433,the insulating layer 435, and the interlayer insulating layer 437, andin the opening, the electrode layer 439 is provided. These steps can beperformed in a manner similar to the formation steps of the openingsreaching the high-concentration impurity regions 409, and the electrodelayers 416.

Lastly, the wiring layer 441 is formed over the interlayer insulatinglayer 437 to be in contact with the electrode layer 439. The wiringlayer 441 can be formed by a step similar to that of the wiring layer423.

Through the above-described steps, the semiconductor device 400 can befabricated.

Since the oxide semiconductor layer is surrounded by (or embedded in)the insulating layer, the transistor 100 can be referred to as atransistor having a trench structure. The transistor 401 has a trenchstructure (shallow trench isolation: STI) in which the transistor issurrounded by the element isolation insulating layers 417. In otherwords, the semiconductor device 400, which includes the transistor 100and the transistor 401, can be referred to as a semiconductor devicehaving two kinds of trench structures (also called double trenchstructure).

It is also possible to fabricate a display device by combining a displayelement with the transistor 100 described in the above embodiment. Forexample, a pixel of a display device can be formed by combining adisplay element and the transistor 100 connected to the display element.A display element, a display device which is a device including adisplay element, a light-emitting element, and a light-emitting devicewhich is a device including a light-emitting element, for example, canemploy various modes and can include various elements. For example, adisplay medium, whose contrast, luminance, reflectivity, transmittance,or the like changes by electromagnetic action, such as an EL(electroluminescence) element (e.g., an EL element including organic andinorganic materials, an organic EL element, or an inorganic EL element),an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), atransistor (a transistor which emits light depending on the amount ofcurrent), an electron emitter, a liquid crystal element, electronic ink,an electrophoretic element, a grating light valve (GLV), a plasmadisplay panel (PDP), a digital micromirror device (DMD), a piezoelectricceramic display, or a carbon nanotube, can be used as a display element,a display device, a light-emitting element, or a light-emitting device.Note that examples of display devices having EL elements include an ELdisplay. Examples of display devices having electron emitters include afield emission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like. Examples ofdisplay devices including liquid crystal elements include a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of display devices having electronic ink orelectrophoretic elements include electronic paper.

[Structural Example 2 of Semiconductor Device]

A semiconductor device of one embodiment of the present invention atleast includes the transistor 100 described in the above embodiment, anda transistor provided below the transistor 100 is not limited to thetransistor 401. A semiconductor device of one embodiment of the presentinvention whose structure is partly different from that of thesemiconductor device 400 is described below.

FIG. 16A is a circuit diagram illustrating a semiconductor device 450whose structure is partly different from that of the semiconductordevice 400, and FIG. 16B illustrates a cross-sectional structure of thesemiconductor device 450.

The semiconductor device 450 is provided with a transistor 451 that is ap-channel transistor in addition to the transistor 401, and includes acomplementary metal oxide semiconductor (CMOS) circuit 452 in which thetransistor 401 and the transistor 451 are electrically connected. In thesemiconductor device 450, the transistor 100 and the capacitor 150 arestacked over the CMOS circuit 452 with the interlayer insulating layersprovided therebetween.

Since the transistor 100 is provided, electric charge can be heldbetween the source or the drain of the transistor 100, one electrode ofthe capacitor 150, and the gates of the transistors 401 and 451 in thesemiconductor device 450, as in the semiconductor device 400. In otherwords, the semiconductor device 450 can function as a semiconductormemory device.

In the semiconductor device 450, since the transistor 100 is included,power consumption is small as compared to the case where thesemiconductor device illustrated in the circuit diagram in FIG. 16A isall formed using a semiconductor material other than an oxidesemiconductor.

Since the semiconductor device 450 and the semiconductor device 400 aredifferent from each other in mainly structures other than the structuresof the transistor 100 and the capacitor 150, the CMOS circuit 452 isdescribed here. Note that in description of the semiconductor device450, the reference numerals used for the semiconductor device 400 areappropriately used.

In the CMOS circuit 452, the transistor 401 and the transistor 451 areelectrically connected as described above.

For details of the transistor 401, the above description can be referredto.

The transistor 451 is provided on an n-well 453 which is formed byaddition of an impurity element imparting n-type conductivity to thesubstrate 403. The transistor 451 includes a channel formation region454 which is provided in the n-well 453; impurity regions 456 betweenwhich the channel formation region 454 is sandwiched; high-concentrationimpurity regions 458 which are electrically connected to the impurityregions 456 (the impurity regions 456 and the high-concentrationimpurity regions 458 are also referred to as impurity regionscollectively); a gate insulating layer 460 which is provided over thechannel formation region 454; a gate electrode layer 462 which isprovided over the gate insulating layer 460; and sidewall insulatinglayers 464 which are provided on side surfaces of the gate electrodelayer 462.

The insulating layer 419 is provided over the transistor 401 and thetransistor 451, and the interlayer insulating layer 421 is provided overthe insulating layer 419. An opening reaching the high-concentrationimpurity region 458 is provided in the insulating layer 419 and theinterlayer insulating layer 421. In the opening, a source electrodelayer or a drain electrode layer (hereinafter referred to as anelectrode layer 447) of the transistor 451 is provided.

The wiring layer 423 is provided in contact with the electrode layer447. The wiring layer 423 is provided in contact with the sourceelectrode layer or the drain electrode layer to function as a sourcewiring or a drain wiring. The wiring layer 423 is electrically connectedto other elements which are included in the semiconductor device 450,other devices, or the like.

In the case of the semiconductor device 450, the element isolationinsulating layers 417 are provided on the substrate 403 so as tosurround the transistor 401 and the transistor 451.

The impurity region 456 functions as an LDD region or an extensionregion. The high-concentration impurity region 458 functions as a sourceregion or a drain region of the transistor 451.

In the semiconductor device 450, electrode layers 466 are provided incontact with the high-concentration impurity region 409 which is closerto the transistor 451 in the transistor 401 and the high-concentrationimpurity region 458 which is closer to the transistor 401 in thetransistor 451. The electrode layers 466 function as the sourceelectrode layer or the drain electrode layer of the transistor 401 andthe source electrode layer or the drain electrode layer of thetransistor 451. Further, the transistor 401 and the transistor 451 areelectrically connected to each other with the electrode layers 466 toform the CMOS circuit 452.

The semiconductor device 450 can be fabricated by using the method forfabricating the semiconductor device 400 as appropriate. Here, a methodfor fabricating the CMOS circuit 452 is described.

An impurity element imparting n-type conductivity is added to a regionof the substrate 403 in which the transistor 451 is to be formed, sothat the n-well 453 is formed in the region. The channel formationregion 454 of the transistor 451 is formed in the n-well 453. As theimpurity element imparting n-type conductivity, phosphorus (P), arsenic(As), or the like can be used. The n-well 453 can be formed by additionof any of the above-described impurity elements by an ion doping method,an ion implantation method, or the like.

Then, the element isolation insulating layers 417 are formed, and thegate insulating layer 411, the gate electrode layer 413, the gateinsulating layer 460, and the gate electrode layer 462 are formed. Forthese steps, the fabrication method of the semiconductor device 400 canbe referred to.

Next, a protective layer is formed over a region where the transistor401 and the element isolation insulating layers 417 are to be formed. Animpurity element imparting p-type conductivity is added to the regionwhere the transistor 451 is to be formed, using the protective layer andthe gate electrode layer 462 as masks, so that impurity regions areformed. After the protective layer is removed, a protective layer isformed over a region where the transistor 451 and the element isolationinsulating layers 417 are to be formed. An impurity element impartingn-type conductivity is added to the region where the transistor 401 isto be formed, using the protective layer and the gate electrode layer413 as masks, so that impurity regions are formed. The concentration ofthe added impurity can be set as appropriate; the concentration ispreferably raised in accordance with the degree of miniaturization ofthe semiconductor element. Alternatively, the impurity element may beadded through an insulating film which is formed to cover the gateinsulating layer 411, the gate electrode layer 413, the gate insulatinglayer 460, and the gate electrode layer 462.

Next, the sidewall insulating layers 415 and the sidewall insulatinglayers 464 are formed. For this step, the fabrication method of thesemiconductor device 400 can be referred to.

Then, an insulating film to be processed into the insulating layer 419is formed. A protective layer is formed over the transistor 401 and theelement isolation insulating layers 417, and an impurity elementimparting p-type conductivity is added to the impurity regions using theprotective layer, the gate electrode layer 462, and the sidewallinsulating layers 464 as masks, whereby the impurity regions 456 and thehigh-concentration impurity regions 458 are formed. After the protectivelayer is removed, a protective layer is formed over the transistor 451and the element isolation insulating layers 417, and an impurity elementimparting n-type conductivity is added to the impurity regions using theprotective layer, the gate electrode layer 413, and the sidewallinsulating layers 415 as masks, whereby the impurity regions 407 and thehigh-concentration impurity regions 409 are formed. Note that theimpurity element may be added before the insulating film to be processedinto the insulating layer 419 is formed, and then the insulating filmmay be formed. Note that the protective layer is removed after theimpurity regions are formed.

Next, the insulating layer 419 and the interlayer insulating layer 421are formed. The electrode layer 416 and the electrode layer 447 whichare respectively in contact with the high-concentration impurity region409 and the high-concentration impurity region 458 are formed. Then, thewiring layer 423 is formed. For these steps, the fabrication method ofthe semiconductor device 400 can be referred to.

After that, the interlayer insulating layer 425 is formed, and thewiring layer 427 is formed. The wiring layer 427 is electricallyconnected to the gate electrode layer 413 of the transistor 401 and thegate electrode layer 462 of the transistor 451 through openings providedin the interlayer insulating layer 425, the interlayer insulating layer421, and the insulating layer 419.

The subsequent steps can be performed as in the case of thesemiconductor device 400.

Through the above-described steps, the semiconductor device 450 can befabricated. The semiconductor device 450, which includes the transistors401 and 451 with trench structures in addition to the transistor 100,can be referred to as a semiconductor device having a double trenchstructure.

[Structural Example 3 of Semiconductor Device]

A semiconductor device of one embodiment of the present invention whosestructure is partly different from that of the semiconductor device 400or the semiconductor device 450 is described below.

FIG. 17 is a circuit diagram illustrating a semiconductor device 480whose structure is partly different from that of the semiconductordevice 400 or the semiconductor device 450.

In the semiconductor device 480, the transistor 100 and the capacitor150 are stacked over a transistor 481 with interlayer insulating layersprovided therebetween.

Since the semiconductor device 480 includes the transistor 100 with areduced off-state current, the power consumption can be reduced.

The element isolation insulating layers 417 are provided on thesubstrate 403 over which the transistor 481 is formed. Impurity regions483, between which a channel formation region 482 is sandwiched, areprovided between the element isolation insulating layers 417. A gateinsulating layer 484 is provided over the channel formation region 482.A first gate electrode layer 485 is provided over the gate insulatinglayer 484. An insulating layer 486 is provided over the first gateelectrode layer 485. A second gate electrode layer 487 is provided overthe insulating layer 486. Sidewall insulating layers 488 are provided onside surfaces of the gate insulating layer 484, the first gate electrodelayer 485, the insulating layer 486, and the second gate electrode layer487.

The insulating layer 419 is provided over the transistor 481. Theinterlayer insulating layer 421 is provided over the insulating layer419. Electrode layers 489 are provided in the insulating layer 419 andthe interlayer insulating layer 421 to be in contact with the impurityregions 483. The wiring layers 423 are provided in contact with theelectrode layers 489.

The interlayer insulating layer 425 is provided over the wiring layers423. The wiring layer 427 is provided over the interlayer insulatinglayer 425. The wiring layer 427 is electrically connected to otherelements of the semiconductor device 480, such as the transistor 481(including the electrode layer 489 and the wiring layer 423), otherdevices, and the like.

The components provided over the wiring layer 427 are the same as thoseof the semiconductor device 400 and the semiconductor device 450.

In the transistor 481, the first gate electrode layer 485 functions as afloating gate; thus, the transistor 481 can function as a nonvolatilememory device. As illustrated in FIG. 17, a plurality of the transistors481 can be provided over the substrate 403. When a plurality of thetransistors 481 are provided, it is possible to increase the amount ofdata which can be stored. Note that in the case where a plurality of thetransistors 481 are provided, the electrode layer 489 is not necessarilyprovided for each of the transistors.

The transistor 481 can be fabricated by appropriately using the methodfor fabricating the transistor 401 or the transistor 451 of thesemiconductor device 400 and the semiconductor device 450. A method forfabricating a transistor which has a floating gate can also be appliedto the fabrication of the transistor 481 as appropriate.

The semiconductor device 480, which includes the transistors 481 withtrench structures in addition to the transistor 100, can be referred toas a semiconductor device having a double trench structure.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

(Embodiment 4)

Any of the semiconductor devices described in the above embodiments canbe applied to a microcomputer used for a variety of electronicappliances.

A structure and operation of a fire alarm system that is an example ofthe electronic appliance using a microcomputer will be described withreference to FIG. 18, FIGS. 19A to 19C, and FIG. 20A.

The fire alarm in this specification refers to any system which raisesan alarm over fire occurrence instantly and includes, for example, aresidential fire alarm, an automatic fire alarm system, and a firedetector used for the automatic fire alarm system in its category.

An alarm system illustrated in FIG. 18 includes at least a microcomputer500. The microcomputer 500 is provided inside the alarm system. Themicrocomputer 500 includes a power gate controller 503 electricallyconnected to a high potential power supply line VDD, a power gate 504electrically connected to the high potential power supply line VDD andthe power gate controller 503, a central processing unit (CPU) 505electrically connected to the power gate 504, and a sensor portion 509electrically connected to the power gate 504 and the CPU 505. Further,the CPU 505 includes a volatile memory portion 506 and a nonvolatilememory portion 507.

The CPU 505 is electrically connected to a bus line 502 via an interface508. The interface 508, as well as the CPU 505, is electricallyconnected to the power gate 504. As a bus standard of the interface 508,for example, an I²C bus can be used. A light-emitting element 530electrically connected to the power gate 504 via the interface 508 isprovided in the alarm system.

The light-emitting element 530 is preferably an element which emitslight with high directivity, and for example, an organic EL element, aninorganic EL element, or an LED can be used.

The power gate controller 503 includes a timer and controls the powergate 504 with the use of the timer. The power gate 504 allows or stopssupply of power from the high potential power supply line VDD to the CPU505, the sensor portion 509, and the interface 508, in accordance withthe control by the power gate controller 503. Here, as an example of thepower gate 504, a switching element such as a transistor can be given.

With the use of the power gate controller 503 and the power gate 504,power is supplied to the sensor portion 509, the CPU 505, and theinterface 508 in a period during which the amount of light is measured,and supply of power to the sensor portion 509, the CPU 505, and theinterface 508 can be stopped during an interval between measurementperiods. The alarm system operates in such a manner, whereby powerconsumption can be reduced as compared to the case where power iscontinuously supplied to the above structures.

In the case where a transistor is used as the power gate 504, it ispreferable to use a transistor which has an extremely low off-statecurrent and can be used for the nonvolatile memory portion 507, forexample, any of the transistors described in the above embodiments. Withthe use of such a transistor, a leakage current can be reduced whensupply of power is stopped by the power gate 504, so that powerconsumption can be reduced.

A direct-current power source 501 may be provided in the alarm system sothat power is supplied from the direct-current power source 501 to thehigh potential power supply line VDD. An electrode of the direct-currentpower source 501 on the high potential side is electrically connected tothe high potential power supply line VDD, and an electrode of thedirect-current power source 501 on the low potential side iselectrically connected to a low potential power supply line VSS. The lowpotential power supply line VSS is electrically connected to themicrocomputer 500. Here, the high potential power supply line VDD issupplied with a high potential H. The low potential power supply lineVSS is supplied with a low potential L, for example, a ground potential(GND).

In the case where a battery is used as the direct-current power source501, for example, a battery case which includes an electrodeelectrically connected to the high potential power supply line VDD, anelectrode electrically connected to the low potential power supply lineVSS, and a housing which can hold the battery is provided in thehousing. Note that the alarm system does not necessarily include thedirect-current power source 501 and may have, for example, a structurein which power is supplied from an alternate-current power sourceprovided outside the alarm system through a wiring.

As the above battery, a secondary battery such as a lithium ionsecondary battery (also called a lithium ion storage battery or alithium ion battery) can be used. Further, a solar battery is preferablyprovided so that the secondary battery can be charged.

The sensor portion 509 measures a physical quantity relating to anabnormal situation and transmits a measured value to the CPU 505. Aphysical quantity relating to an abnormal situation depends on the usageof the alarm system, and in an alarm system functioning as a fire alarm,a physical quantity relating to a fire is measured. Thus, the sensorportion 509 measures the amount of light as a physical quantity relatingto a fire and senses smoke.

The sensor portion 509 includes an optical sensor 511 electricallyconnected to the power gate 504, an amplifier 512 electrically connectedto the power gate 504, and an AD converter 513 electrically connected tothe power gate 504 and the CPU 505. The light-emitting element 530, theoptical sensor 511, the amplifier 512, and the AD converter 513 operatewhen the power gate 504 allows supply of power to the sensor portion509.

The optical sensor 511 includes at least a photoelectric conversionelement such as a photodiode. The optical sensor 511 can be fabricatedby utilizing the fabrication process of any of the semiconductor devices(e.g., the semiconductor device 400, the semiconductor device 450, andthe semiconductor device 480) which are described in the aboveembodiments.

The photoelectric conversion element can be fabricated with the use of asemiconductor film which can perform photoelectric conversion, and forexample, silicon, germanium, or the like can be used. In the case ofusing silicon for the semiconductor film, an optical sensor which sensesvisible light can be obtained. Further, there is a difference betweensilicon and germanium in wavelengths of absorbed electromagnetic waves.In the case of using germanium for the semiconductor film, a sensorwhich senses infrared rays can be obtained.

In the above manner, the sensor portion 509 including the optical sensor511 can be incorporated into the microcomputer 500, so that the numberof components can be reduced and the housing of the alarm system can bereduced.

In the fire alarm including the above-described IC chip, the CPU 505 inwhich a plurality of circuits each including any of the abovetransistors are combined and mounted on one IC chip is used.

FIGS. 19A to 19C are block diagrams illustrating a specific structure ofa CPU at least partly including any of the semiconductor devicesdescribed in the above embodiments.

The CPU illustrated in FIG. 19A includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and a ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may be provided over separate chips. Needless to say, the CPU inFIG. 19A is just an example of a simplified structure, and an actual CPUmay have a variety of structures depending on the application.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 on the basis of areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 19A, a memory cell is provided in theregister 1196. As the memory cell of the register 1196, the abovetransistor can be used.

In the CPU illustrated in FIG. 19A, the register controller 1197 selectsan operation of holding data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of a powersupply voltage to the memory cell in the register 1196 can be stopped.

The power supply can be stopped by a switching element provided betweena memory cell group and a node to which a power supply potential VDD ora power supply potential VSS is supplied, as illustrated in FIG. 19B orFIG. 19C. Circuits illustrated in FIGS. 19B and 19C are described below.

FIGS. 19B and 19C each illustrate a memory device in which any of thesemiconductor devices described in the above embodiments is used as aswitching element which controls supply of a power supply potential to amemory cell.

The memory device illustrated in FIG. 19B includes a switching element1141 and a memory cell group 1143 including a plurality of memory cells1142. Specifically, as each of the memory cells 1142, the abovetransistor can be used. Each of the memory cells 1142 included in thememory cell group 1143 is supplied with the high-level power supplypotential VDD via the switching element 1141. Further, each of thememory cells 1142 included in the memory cell group 1143 is suppliedwith a potential of a signal IN and the low-level power supply potentialVSS.

In FIG. 19B, the above transistor is used as the switching element 1141,and the switching of the transistor is controlled by a signal SigAsupplied to a gate electrode layer thereof.

Note that FIG. 19B illustrates the structure in which the switchingelement 1141 includes only one transistor; however, one embodiment ofthe present invention is not limited thereto and the switching element1141 may include a plurality of transistors. In the case where theswitching element 1141 includes a plurality of transistors which serveas switching elements, the plurality of transistors may be connected toeach other in parallel, in series, or in combination of parallelconnection and series connection.

Although the switching element 1141 controls the supply of thehigh-level power supply potential VDD to each of the memory cells 1142included in the memory cell group 1143 in FIG. 19B, the switchingelement 1141 may control the supply of the low-level power supplypotential VSS.

FIG. 19C illustrates an example of a memory device in which each of thememory cells 1142 included in the memory cell group 1143 is suppliedwith the low-level power supply potential VSS via the switching element1141. The supply of the low-level power supply potential VSS to each ofthe memory cells 1142 included in the memory cell group 1143 can becontrolled by the switching element 1141.

When a switching element is provided between a memory cell group and anode to which the power supply potential VDD or the power supplypotential VSS is supplied, data can be held even in the case where anoperation of a CPU is temporarily stopped and the supply of the powersupply voltage is stopped; accordingly, power consumption can bereduced. Specifically, for example, while a user of a personal computerdoes not input data to an input device such as a keyboard, the operationof the CPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example, the transistor can also beapplied to an LSI such as a digital signal processor (DSP), a customLSI, or a field programmable gate array (FPGA).

In FIG. 20A, an alarm system 8100 is a residential fire alarm, whichincludes a sensor portion and a microcomputer 8101. The microcomputer8101 includes a CPU in which any of the above transistors is used.

In FIG. 20A, a CPU that uses any of the above transistors is included inan air conditioner which includes an indoor unit 8200 and an outdoorunit 8204. Specifically, the indoor unit 8200 includes a housing 8201,an air outlet 8202, a CPU 8203, and the like. Although the CPU 8203 isprovided in the indoor unit 8200 in FIG. 20A, the CPU 8203 may beprovided in the outdoor unit 8204. Alternatively, the CPU 8203 may beprovided in both the indoor unit 8200 and the outdoor unit 8204. A CPUthat uses any of the above transistors can save the power of the airconditioner.

In FIG. 20A, a CPU that uses any of the above transistors is included inan electric refrigerator-freezer 8300. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, a CPU 8304, and the like.In FIG. 20A, the CPU 8304 is provided in the housing 8301. A CPU thatuses any of the above transistors can save the power of the electricrefrigerator-freezer 8300.

FIGS. 20B and 20C illustrate an example of an electric vehicle. Theelectric vehicle 9700 is equipped with a secondary battery 9701. Theoutput of the electric power of the secondary battery 9701 is adjustedby a control circuit 9702 and the electric power is supplied to adriving device 9703. The control circuit 9702 is controlled by aprocessing unit 9704 including a ROM, a RAM, a CPU, or the like which isnot illustrated. A CPU that uses any of the above transistors can savethe power of the electric vehicle 9700.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the control circuit 9702 on thebasis of input data such as data of operation (e.g., acceleration,deceleration, or stop) by a driver or data during driving (e.g., data onan upgrade or a downgrade, or data on a load on a driving wheel) of theelectric vehicle 9700. The control circuit 9702 adjusts the electricenergy supplied from the secondary battery 9701 in accordance with thecontrol signal of the processing unit 9704 to control the output of thedriving device 9703. In the case where the AC motor is mounted, althoughnot illustrated, an inverter which converts a direct current into analternate current is also incorporated.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

This application is based on Japanese Patent Application serial no.2012-264583 filed with Japan Patent Office on Dec. 3, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a first oxidelayer over an insulating surface; an oxide semiconductor layer over thefirst oxide layer; a second oxide layer over the oxide semiconductorlayer; an insulating layer surrounding a side surface of the oxidesemiconductor layer; a source electrode and a drain electrode in contactwith a top surface of the insulating layer; a gate electrode overlappingwith the oxide semiconductor layer; and a gate insulating layer betweenthe oxide semiconductor layer and the gate electrode, wherein the sourceelectrode and the drain electrode are electrically connected to theoxide semiconductor layer, wherein a side surface of the first oxidelayer and the side surface of the oxide semiconductor layer aresubstantially perpendicular to the insulating surface, and wherein thetop surface of the insulating layer is planarized, wherein the oxidesemiconductor layer comprises a first region in electrical contact withthe source electrode, wherein the oxide semiconductor layer comprises asecond region in electrical contact with the drain electrode, whereinthe oxide semiconductor layer comprises a third region comprising achannel, wherein the third region is provided between the first regionand the second region, wherein a width of the first oxide layer in achannel length direction is greater than a width of the third region ofthe oxide semiconductor layer in the channel length direction, wherein awidth of the third region of the oxide semiconductor layer in thechannel length direction is greater than a width of the second oxidelayer in the channel length direction, and wherein each of an endportion of the source electrode and an end portion of the drainelectrode is interposed between the oxide semiconductor layer and thesecond oxide layer.
 2. The semiconductor device according to claim 1,further comprising an oxide layer between the oxide semiconductor layerand the insulating layer, wherein the oxide layer is in contact with theside surface of the oxide semiconductor layer.
 3. The semiconductordevice according to claim 2, wherein the oxide layer comprises one ormore kinds of metal elements contained in the oxide semiconductor layer.4. The semiconductor device according to claim 1, wherein the sourceelectrode comprises a first source electrode and a second sourceelectrode over the first source electrode, wherein the drain electrodecomprises a first drain electrode and a second drain electrode over thefirst drain electrode, wherein the first source electrode and the firstdrain electrode are in direct contact with the oxide semiconductor layerand the insulating layer, and wherein the second source electrode andthe second drain electrode are in direct contact with the oxidesemiconductor layer.
 5. A semiconductor device comprising: a first oxidelayer over an insulating surface, an oxide semiconductor layer over thefirst oxide layer: a second oxide layer over the oxide semiconductorlayer; an insulating layer surrounding a side surface of the oxidesemiconductor layer; an oxide layer between the oxide semiconductorlayer and the insulating layer; a source electrode and a drain electrodein contact with a top surface of the insulating layer; a gate electrodeoverlapping with the oxide semiconductor layer; and a gate insulatinglayer between the oxide semiconductor layer and the gate electrode,wherein the source electrode and the drain electrode are electricallyconnected to the oxide semiconductor layer, wherein a side surface ofthe first oxide layer and the side surface of the oxide semiconductorlayer are substantially perpendicular to the insulating surface, andwherein the oxide layer is in contact with the side surface of the oxidesemiconductor layer, wherein the oxide semiconductor layer comprises afirst region in electrical contact with the source electrode, whereinthe oxide semiconductor layer comprises a second region in electricalcontact with the drain electrode, wherein the oxide semiconductor layercomprises a third region comprising a channel, wherein the third regionis provided between the first region and the second region, wherein awidth of the first oxide layer in a channel length direction is greaterthan a width of the third region of the oxide semiconductor layer in thechannel length direction, wherein a width of the third region of theoxide semiconductor layer in the channel length direction is greaterthan a width of the second oxide layer in the channel length direction,and wherein each of an end portion of the source electrode and an endportion of the drain electrode is interposed between the oxidesemiconductor layer and the second oxide layer.
 6. The semiconductordevice according to claim 5, wherein the oxide layer comprises one ormore kinds of metal elements contained in the oxide semiconductor layer.7. The semiconductor device according to claim 5, wherein the sourceelectrode comprises a first source electrode and a second sourceelectrode over the first source electrode, wherein the drain electrodecomprises a first drain electrode and a second drain electrode over thefirst drain electrode, wherein the first source electrode and the firstdrain electrode are in direct contact with the oxide semiconductor layerand the insulating layer, and wherein the second source electrode andthe second drain electrode are in direct contact with the oxidesemiconductor layer.
 8. The semiconductor device according to claim 1,wherein the second oxide layer is in contact with a top surface of theoxide semiconductor layer, a top surface of the source electrode, and atop surface of the drain electrode.
 9. The semiconductor deviceaccording to claim 5, wherein the second oxide layer is in contact witha top surface of the oxide semiconductor layer, a top surface of thesource electrode, and a top surface of the drain electrode.
 10. Thesemiconductor device according to claim 1, wherein a thickness of thethird region is thinner than a thickness of each of the first region andthe second region.
 11. The semiconductor device according to claim 5,wherein a thickness of the third region is thinner than a thickness ofeach of the first region and the second region.